Fix memory leak in guiMutexLock() function (flameshot-org#4277)

- Add proper cleanup when QSharedMemory::create() fails
- Prevents memory leak when mutex creation fails on GUI capture attempts
- Ensures allocated QSharedMemory object is deleted before returning nullptr

Fixes potential memory leak identified in codebase analysis.

Author: avineet4

src/main.cpp

@@ -101,6 +101,7 @@ QSharedMemory* guiMutexLock()
     shm = new QSharedMemory(key);
 #endif
     if (!shm->create(1)) {
+        delete shm;
         return nullptr;
     }
     return shm;