Avoid undefined behavior in probable_prime.

(Imported from upstream's e4676e900f165f5272991443225813002300b09b.)

Change-Id: I678e158c223daf2f7f9114f4e743d531fe2e2a93
Reviewed-on: https://boringssl-review.googlesource.com/4044
Reviewed-by: Adam Langley <agl@google.com>
1 file changed