Check the return value of |BN_sub| in bn_test.cc.

Coverity complains about this.

Change-Id: I5140fd82bdbe7c2e3b36a8bc9006770a431741ea
Reviewed-on: https://boringssl-review.googlesource.com/5619
Reviewed-by: Adam Langley <agl@google.com>
1 file changed