Remove superfluous “!= 0” in if.

“!= 0” is implicit in if statements and it looks very weird here.

Change-Id: I7f4e71c479b8ff9821a040f1c542b15af19b8aed
Reviewed-on: https://boringssl-review.googlesource.com/5720
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>