oops, forgot to create the mouse access mutex

src/unix/unix.c

@@ -581,7 +581,7 @@ int main(int argc, char** argv)
         fprintf(stderr, "Failed to create blit mutex: %s", SDL_GetError());
         return -1;
     }
-
+    mousemutex = SDL_CreateMutex();
     sdl_initho();
 
     if (start_in_fullscreen)