Return NULL instead of zero in |bn_resized_from_ctx|.

Change-Id: I5fc029ceddfa60b2ccc97c138b94c1826f6d75fa
Reviewed-on: https://boringssl-review.googlesource.com/25844
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

crypto/fipsmodule/bn/div.c

@@ -481,7 +481,7 @@ static const BIGNUM *bn_resized_from_ctx(const BIGNUM *bn, size_t width,
   if (ret == NULL ||
       !BN_copy(ret, bn) ||
       !bn_resize_words(ret, width)) {
-    return 0;
+    return NULL;
   }
   return ret;
 }