fix: multirepo potential nil pointer dereference #1074

Job	Run time
examples / client (ubuntu-latest)	18s
examples / multirepo (ubuntu-latest)	18s
examples / client (macos-latest)	21s
examples / multirepo (macos-latest)	20s
examples / client (windows-latest)	1m 51s
examples / multirepo (windows-latest)	51s
examples / repository (ubuntu-latest)	14s
examples / root-signing (ubuntu-latest)	17s
examples / repository (macos-latest)	21s
examples / root-signing (macos-latest)	20s
examples / repository (windows-latest)	1m 39s
examples / root-signing (windows-latest)	55s
examples / tuf-client-cli (ubuntu-latest)	9s
linting / golangci-lint	2m 33s
examples / tuf-client-cli (macos-latest)	22s
examples / tuf-client-cli (windows-latest)	1m 9s
linting / govulncheck	15s
tests / Run (ubuntu-latest)	36s
tests / Run (macos-latest)	33s
tests / Run (windows-latest)	2m 24s
	15m 46s

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