Fix signed/unsigned warning in bn_test.cc.

BN_num_bits returns unsigned.

Change-Id: Ie346dbe0a12d3597d2b12e56b57dfc147e65dcc3
Reviewed-on: https://boringssl-review.googlesource.com/4895
Reviewed-by: Adam Langley <agl@google.com>
1 file changed