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/window.c

@@ -830,9 +830,9 @@ WWindow *wManageWindow(WScreen *scr, Window window)
 			wwin->main_window = fPtr->leader;
 		}
 		if (instance)
-			XFree(instance);
+			free(instance);
 		if (class)
-			XFree(class);
+			free(class);
 #undef ADEQUATE
 	}
 
@@ -2654,9 +2654,9 @@ WMagicNumber wWindowGetSavedState(Window win)
 	if (command)
 		wfree(command);
 	if (instance)
-		XFree(instance);
+		free(instance);
 	if (class)
-		XFree(class);
+		free(class);
 
 	return wstate;
 }