streams: openssl: ignore return value of `git_mutex_lock`

OpenSSL pre-v1.1 required us to set up a locking function to properly
support multithreading. The locking function signature cannot return any
error codes, and as a result we can't do anything if `git_mutex_lock`
fails. To silence static analysis tools, let's just explicitly ignore
its return value by casting it to `void`.

src/streams/openssl.c

@@ -160,7 +160,7 @@ static void openssl_locking_function(
 	lock = mode & CRYPTO_LOCK;
 
 	if (lock) {
-		git_mutex_lock(&openssl_locks[n]);
+		(void)git_mutex_lock(&openssl_locks[n]);
 	} else {
 		git_mutex_unlock(&openssl_locks[n]);
 	}