MACOSX: Improve conversion of path from NSString to C string

Use `-[NSString fileSystemRepresentation]` if you are going to pass
a path to a lower-level function, like `fopen()`.

backends/platform/sdl/macosx/macosx_wrapper.mm

@@ -108,5 +108,5 @@ Common::String getDesktopPathMacOSX() {
 	NSString *path = [paths objectAtIndex:0];
 	if (path == nil)
 		return Common::String();
-	return Common::String([path cStringUsingEncoding:NSASCIIStringEncoding]);
+	return Common::String([path fileSystemRepresentation]);
 }