Work around another NULL/0 C language bug.

memchr should be OPENSSL_memchr to avoid tripping the memchr variant
of the memcpy(NULL, NULL, 0) language flaw.

Change-Id: I5c945fa6df026d56073f3c5839c0d19cecaf1aa0
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/39624
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/crypto/fipsmodule/rsa/rsa_impl.c b/crypto/fipsmodule/rsa/rsa_impl.c
index caa2eb4..94fb75c 100644
--- a/crypto/fipsmodule/rsa/rsa_impl.c
+++ b/crypto/fipsmodule/rsa/rsa_impl.c
@@ -368,7 +368,7 @@
   CRYPTO_MUTEX_lock_write(&rsa->lock);
 
   uint8_t *const free_inuse_flag =
-      memchr(rsa->blindings_inuse, 0, rsa->num_blindings);
+      OPENSSL_memchr(rsa->blindings_inuse, 0, rsa->num_blindings);
   if (free_inuse_flag != NULL) {
     *free_inuse_flag = 1;
     *index_used = free_inuse_flag - rsa->blindings_inuse;