Fix DER checks for DSA_check_signature and add tests.

DSA_verify and DSA_check_signature didn't share a codepath, so the fix was only
applied to the former. Implement verify in terms of check_signature and add
tests for bad DER variants.

Change-Id: I6577f96b13b57fc89a5308bd8a7c2318defa7ee1
Reviewed-on: https://boringssl-review.googlesource.com/2820
Reviewed-by: Adam Langley <agl@google.com>
3 files changed