OpenSSL's BN_mul function had a single-word buffer underflow (see
576129cd72). We already independently
fixed this but, if we hadn't, ASan wouldn't have noticed because of
OPENSSL_malloc.
ASan has runtime hooks we can call to make it more accurate.
Change-Id: Ifc9c3837ece2bc456c5bdc960be707d7b1759904
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/35165
Reviewed-by: Adam Langley <agl@google.com>