Plug some (possible) memleaks

PropGetWMClass()

- XAllocClassHint()s a struct for class hint data
- This is filled by XGetClassHint(), which in turn uses Xlib to allocate
  some more space for XClassHint members
- Upon XGetClassHint() failure, "default" is libc malloc'd (via strdup),
  and is returned to the caller
- Upon XGetClassHint() success, XClassHint members are returned raw --
  these members must be freed with XFree() (see XAllocClassHint(3))
- Thus it's up to PropGetWMClass() callers to decide (based upon the return
  value) which method (libc free() or XFree()) to use to free res_name
  and res_class. This was done nowhere, thus leaking some memory
  on every failed PropGetWMClass() call.
- So just strdup the successful res_name/res_class members, XFree() them
  while still in PropGetWMClass(), and allow callers to unconditionally
  libc free() whatever PropGetWMClass() returns.

src/dock.c

@@ -3015,9 +3015,9 @@ void wDockTrackWindowLaunch(WDock * dock, Window window)
 		wfree(command);
 
 	if (wm_class)
-		XFree(wm_class);
+		free(wm_class);
 	if (wm_instance)
-		XFree(wm_instance);
+		free(wm_instance);
 }
 
 void wClipUpdateForWorkspaceChange(WScreen * scr, int workspace)