cmd/cue: add a test case for issue 4286
This should give a useful error message rather than panic.
For #4286.
Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I556e05623d5a232f9cdef775052d6f292c690a8c
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1232121
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Matthew Sackman <matthew@cue.works>