With fixed-width BIGNUMs, this is no longer a concern. With this CL, I
believe we now no longer call BN_num_bits on BIGNUMs with secret
magnitude.

Of course, DSA then turns around and calls the variable-time BN_mod
immediately afterwards anyway. But the DSA is deprecated and doomed to
be removed someday anyway.

Change-Id: Iac1dab22aa51c0e7f5ca0f7f44a026a242a4eaa2
Reviewed-on: https://boringssl-review.googlesource.com/25284
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>