Optimize mkpath()

If the path already exists (is a directory), don't need to check each path
element for existence.

This should help with the speed of saving of large file.ini (Nelgin's) when a
lot of directories have "Check for File Existence" set to "Yes".

src/xpdev/dirwrap.c

@@ -1214,6 +1214,9 @@ int mkpath(const char* path)
 	char	dir[MAX_PATH+1];
 	int		result=0;
 
+	if(isdir(path))
+		return 0;
+
 #ifdef _WIN32
 	if(p[1]==':')	/* Skip drive letter, if specified */
 		p+=2;