Fix panic in getWebConfig

[rosstimothy]

Refactored the usage of the types.AuthPreference returned from
GetAuthPreference so that it is only accessed if there were no
errors.

runtime error: invalid memory address or nil pointer dereference
goroutine 795303 [running]:
net/http.(*conn).serve.func1()
/opt/go/src/net/http/server.go:1801 +0xb9
panic({0x3cd77a0, 0x756dee0})
/opt/go/src/runtime/panic.go:1047 +0x266
github.com/gravitational/teleport/lib/web.(*Handler).getWebConfig(0xc000840500, {0x52618a0, 0xc0001f09a0}, 0xc000755400, {0xc001022240, 0x4262ed2, 0xc000b5bc80})
/go/src/github.com/gravitational/teleport/lib/web/apiserver.go:894 +0x643
github.com/gravitational/teleport/lib/httplib.MakeHandlerWithErrorWriter.func1({0x52618a0, 0xc0001f09a0}, 0xc001190920, {0x0, 0x0, 0x0})
/go/src/github.com/gravitational/teleport/lib/httplib/httplib.go:67 +0x89
github.com/julienschmidt/httprouter.(*Router).ServeHTTP(0xc000840518, {0x52618a0, 0xc0001f09a0}, 0xc000755400)
/go/src/github.com/gravitational/teleport/vendor/github.com/julienschmidt/httprouter/router.go:387 +0x84b
github.com/gravitational/teleport/lib/web.(*WebAPIHandler).ServeHTTP(0xc000bdb860, {0x52618a0, 0xc0001f09a0}, 0xc000755400)
/go/src/github.com/gravitational/teleport/lib/web/apiserver.go:215 +0x136
github.com/gravitational/oxy/ratelimit.(*TokenLimiter).ServeHTTP(0xc00027b100, {0x52618a0, 0xc0001f09a0}, 0xc000755400)
go/src/github.com/gravitational/teleport/vendor/github.com/gravitational/oxy/ratelimit/tokenlimiter.go:118 +0x1ce
github.com/gravitational/oxy/connlimit.(*ConnLimiter).ServeHTTP(0xc000625320, {0x52618a0, 0xc0001f09a0}, 0x10000c001022170)
/go/src/github.com/gravitational/teleport/vendor/github.com/gravitational/oxy/connlimit/connlimit.go:75+0x326
net/http.serverHandler.ServeHTTP({0xc000a17770}, {0x52618a0, 0xc0001f09a0}, 0xc000755400)
/opt/go/src/net/http/server.go:2878 +0x43b
net/http.(*conn).serve(0xc001196960, {0x52a50e0, 0xc0001199b0})
/opt/go/src/net/http/server.go:1929 +0xb08
created by net/http.(*Server).Serve
/opt/go/src/net/http/server.go:3033 +0x4e8" 
utils/cli.go:351

[codingllama]

Thanks!