Fix return value of PHYSFS_readBytes() being stored in a smaller size

PHYSFS_readBytes() returns a PHYSFS_sint64, but we forcefully shove it
into a 32-bit signed integer.

Fixing the type of this doesn't have any immediate consequences, but
it's good for the future in case we want to use the return value for
files bigger than 2 gigabytes; it doesn't harm us in any way, and it's
just better housekeeping.

desktop_version/src/FileSystemUtils.cpp

@@ -386,7 +386,7 @@ void FILESYSTEM_loadFileToMemory(
 			VVV_exit(1);
 		}
 	}
-	int success = PHYSFS_readBytes(handle, *mem, length);
+	PHYSFS_sint64 success = PHYSFS_readBytes(handle, *mem, length);
 	if (success == -1)
 	{
 		FILESYSTEM_freeMemory(mem);