Fix for CVE-2014-3570.

(With minor bn/generic.c revamp.)

(Imported from upstream's 56df92efb6893abe323307939425957ce878c8f0)

Change-Id: I9d85cfde4dfb29e64ff7417f781d0c9f1685e905
Reviewed-on: https://boringssl-review.googlesource.com/2780
Reviewed-by: Adam Langley <agl@google.com>