Regression test for CVE-2015-0291.

This is really just scar tissue with https://crbug.com/468889 being the real
underlying problem. But the test is pretty easy.

Change-Id: I5eca18fdcbde8665c0e6c3ac419a28152647d66f
Reviewed-on: https://boringssl-review.googlesource.com/4052
Reviewed-by: Adam Langley <agl@google.com>