Treat git_mutex_lock as successful when threads are disabled
diff --git a/src/thread-utils.h b/src/thread-utils.h
index f0a51f2..d747a0d 100644
--- a/src/thread-utils.h
+++ b/src/thread-utils.h
@@ -79,7 +79,7 @@ GIT_INLINE(int) git_atomic_dec(git_atomic *a)
/* Pthreads Mutex */
#define git_mutex unsigned int
#define git_mutex_init(a) (void)0
-#define git_mutex_lock(a) (void)0
+#define git_mutex_lock(a) 0
#define git_mutex_unlock(a) (void)0
#define git_mutex_free(a) (void)0