Fix n3ds/SDL_syssem.c:SDL_SignalSemaphore (libsdl-org/SDL#12411)

(cherry picked from commit 0306b5a865)

src/thread/n3ds/SDL_syssem.c

@@ -102,7 +102,7 @@ Uint32 SDL_GetSemaphoreValue(SDL_Semaphore *sem)
 
 void SDL_SignalSemaphore(SDL_Semaphore *sem)
 {
-    if (sem) {
+    if (!sem) {
         return;
     }