ROMManager: initialise filedata to nullptr

If a user manages to open a file as a ROM that is greater than 1 GiB,
it will cause a segmentation fault (a crash) in LoadROM due to a delete
being called on an uninitialised pointer, which is undefined behaviour.

Initialise filedata to nullptr to prevent this, as deleting a null
pointer is defined as a no-op.

src/frontend/qt_sdl/ROMManager.cpp

@@ -1109,7 +1109,7 @@ bool LoadROM(QStringList filepath, bool reset)
 {
     if (filepath.empty()) return false;
 
-    u8* filedata;
+    u8* filedata = nullptr;
     u32 filelen;
 
     std::string basepath;