Add length check to OGG check

Dav999 pointed out this potential issue on Discord.

While basically all memcmp implementations will terminate early here if
there's a null byte (because it's mismatched), it doesn't hurt to add
the check here.

desktop_version/src/Music.cpp

@@ -107,7 +107,7 @@ public:
         }
 
         SDL_zerop(this);
-        if (SDL_memcmp(mem, "OggS", 4) == 0)
+        if (length >= 4 && SDL_memcmp(mem, "OggS", 4) == 0)
         {
             LoadOGG(fileName, mem, length);
         }