cmd/go-contrib-init: don't crash from src/golang.org/x
cmd/go-contrib-init: don't crash from src/golang.org/x

If go-contrib-init is executed from the src/golang.org/x directory (as
opposed to a sub-directory), it crashes with an index out-of-bounds.

Fix this by including the trailing slash in the inspected path prefix
so we never slice more than what we know is in the string.

Change-Id: Ibbb74759c2ba839972de34a8ffb28b6cfb6825e2
Reviewed-on: https://go-review.googlesource.com/48690
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools