fix verify-vendor if go.mod does not contain replace rules
fix verify-vendor if go.mod does not contain replace rules

Before this patch, if the go.mod did not contain a replace rule, jq would fail
(Cannot iterate over null), and Bash would produce an error when trying to add
an empty `$key` to `map_requires`:

    make verify-vendor
    + verify-vendor
      jq: error (at <stdin>:581): Cannot iterate over null (null)
      /go/src/github.com/containerd/containerd//script/verify-go-modules.sh: line 44: map_replaces_1[$key]: bad array subscript
      make: *** [Makefile:435: verify-vendor] Error 1

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

estesp/containerd,containerd/containerd,thaJeztah/containerd,mikebrow/containerd,estesp/containerd,mikebrow/containerd,containerd/containerd,thaJeztah/containerd