services/horizon: Handle bad request to offers, don't throw a 500

[abuiles]

The following request causes a 500

/offers?buying=native\\u0026cursor=\\u0026limit=10\\u0026order=asc\\u0026selling=BTC%3AGAEDZ7BHMDYEMU6IJT3CTTGDUSLZWS5CQWZHGP4XUOIDG5ISH3AFAEK2

[bartekn]

Stack trace:

runtime.boundsError runtime error: index out of range [1] with length 1
/var/lib/jenkins/jobs/stellar-horizon-public-unstable/workspace/services/horizon/internal/actions/query_params.go:97 (0xcaa45b)
/var/lib/jenkins/jobs/stellar-horizon-public-unstable/workspace/services/horizon/internal/actions/offer.go:97 (0xca73b6)
/var/lib/jenkins/jobs/stellar-horizon-public-unstable/workspace/services/horizon/internal/handler.go:494 (0xcef2d7)
/var/lib/jenkins/jobs/stellar-horizon-public-unstable/workspace/services/horizon/internal/handler.go:552 (0xcef850)
/var/lib/jenkins/jobs/stellar-horizon-public-unstable/workspace/vendor/github.com/go-chi/chi/mux.go:425 (0xba9be8)
/usr/lib/go-1.13.3/src/net/http/server.go:2007 (0x81af64)
/var/lib/jenkins/jobs/stellar-horizon-public-unstable/workspace/vendor/github.com/go-chi/chi/mux.go:70 (0xba7a63)
/var/lib/jenkins/jobs/stellar-horizon-public-unstable/workspace/vendor/github.com/go-chi/chi/mux.go:292 (0xbad841)
/usr/lib/go-1.13.3/src/net/http/server.go:2007 (0x81af64)
/usr/lib/go-1.13.3/src/net/http/server.go:2007 (0xd0411d)
/usr/lib/go-1.13.3/src/net/http/server.go:2007 (0xd040f2)
/usr/lib/go-1.13.3/src/net/http/server.go:2007 (0x81af64)
/var/lib/jenkins/jobs/stellar-horizon-public-unstable/workspace/vendor/github.com/go-chi/chi/chain.go:31 (0xba6e42)
/var/lib/jenkins/jobs/stellar-horizon-public-unstable/workspace/vendor/github.com/go-chi/chi/mux.go:425 (0xba9be8)
/usr/lib/go-1.13.3/src/net/http/server.go:2007 (0x81af64)
/var/lib/jenkins/jobs/stellar-horizon-public-unstable/workspace/vendor/github.com/stellar/throttled/http.go:73 (0xc8a9af)
/usr/lib/go-1.13.3/src/net/http/server.go:2007 (0x81af64)
/var/lib/jenkins/jobs/stellar-horizon-public-unstable/workspace/vendor/github.com/rs/cors/cors.go:190 (0xcdabc4)
/usr/lib/go-1.13.3/src/net/http/server.go:2007 (0x81af64)
/var/lib/jenkins/jobs/stellar-horizon-public-unstable/workspace/vendor/github.com/go-chi/chi/middleware/compress.go:190 (0xcd4505)
/usr/lib/go-1.13.3/src/net/http/server.go:2007 (0x81af64)
/var/lib/jenkins/jobs/stellar-horizon-public-unstable/workspace/services/horizon/internal/middleware.go:219 (0xd03803)
/usr/lib/go-1.13.3/src/net/http/server.go:2007 (0x81af64)
/var/lib/jenkins/jobs/stellar-horizon-public-unstable/workspace/services/horizon/internal/middleware.go:230 (0xd038e0)
/var/lib/jenkins/jobs/stellar-horizon-public-unstable/workspace/vendor/github.com/rcrowley/go-metrics/timer.go:227 (0xb965f0)
/var/lib/jenkins/jobs/stellar-horizon-public-unstable/workspace/services/horizon/internal/middleware.go:229 (0xd03a1c)
/usr/lib/go-1.13.3/src/net/http/server.go:2007 (0x81af64)
/var/lib/jenkins/jobs/stellar-horizon-public-unstable/workspace/services/horizon/internal/middleware.go:117 (0xd03384)
/usr/lib/go-1.13.3/src/net/http/server.go:2007 (0x81af64)
/var/lib/jenkins/jobs/stellar-horizon-public-unstable/workspace/services/horizon/internal/middleware.go:93 (0xd02df5)
/usr/lib/go-1.13.3/src/net/http/server.go:2007 (0x81af64)
/var/lib/jenkins/jobs/stellar-horizon-public-unstable/workspace/vendor/github.com/sebest/xff/xff.go:64 (0xcdb7d4)
/usr/lib/go-1.13.3/src/net/http/server.go:2007 (0x81af64)
/var/lib/jenkins/jobs/stellar-horizon-public-unstable/workspace/services/horizon/internal/middleware.go:56 (0xd02907)
/usr/lib/go-1.13.3/src/net/http/server.go:2007 (0x81af64)
/var/lib/jenkins/jobs/stellar-horizon-public-unstable/workspace/vendor/github.com/go-chi/chi/middleware/request_id.go:76 (0xcd4fef)
/usr/lib/go-1.13.3/src/net/http/server.go:2007 (0x81af64)
/var/lib/jenkins/jobs/stellar-horizon-public-unstable/workspace/services/horizon/internal/middleware.go:46 (0xd02751)
/usr/lib/go-1.13.3/src/net/http/server.go:2007 (0x81af64)
/var/lib/jenkins/jobs/stellar-horizon-public-unstable/workspace/services/horizon/internal/middleware.go:34 (0xd0239a)
/usr/lib/go-1.13.3/src/net/http/server.go:2007 (0x81af64)
/var/lib/jenkins/jobs/stellar-horizon-public-unstable/workspace/vendor/github.com/go-chi/chi/middleware/strip.go:25 (0xcd537b)
/usr/lib/go-1.13.3/src/net/http/server.go:2007 (0x81af64)
/var/lib/jenkins/jobs/stellar-horizon-public-unstable/workspace/vendor/github.com/go-chi/chi/mux.go:82 (0xba7802)
/usr/lib/go-1.13.3/src/net/http/server.go:2802 (0x81e3b4)
/usr/lib/go-1.13.3/src/net/http/server.go:1890 (0x819d55)
/usr/lib/go-1.13.3/src/runtime/asm_amd64.s:1357 (0x460a01)