Refactor BN_rand_range to reduce code duplication.
Besides reducing code duplication, also move the relative location of
the check of |count|. Previously, the code was generating a random
value and then terminating the loop without using it if |count| went
to zero. Now the wasted call to |BN_rand| is not made.
Also add a note about the applicability of the special case logic for
|range| of the form |0b100...| to RSA blinding.
Change-Id: Iaa33b9529f1665ac59aefcc8b371fa32445e7578
Reviewed-on: https://boringssl-review.googlesource.com/8960
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>