This was caught by an aarch64 ABI tester. aarch64 has the same
considerations around small arguments as x86_64 does. The aarch64
version of bn_mul_mont does not mask off the upper words of the
argument.
The x86_64 version does, so size_t is, strictly speaking, wrong for
aarch64, but bn_mul_mont already has an implicit size limit to support
its internal alloca, so this doesn't really make things worse than
before.
Change-Id: I39bffc8fdb2287e45a2d1f0d1b4bd5532bbf3868
Reviewed-on: https://boringssl-review.googlesource.com/c/34804
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>
55b9acda99