make sure that mouse.z == 0 until it is used.

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%403634

src/events/SDL_mouse.c

@@ -464,6 +464,7 @@ SDL_SendMouseMotion(int id, int relative, int x, int y, int pressure)
         event.motion.state = mouse->buttonstate;
         event.motion.x = mouse->x;
         event.motion.y = mouse->y;
+        event.motion.z = mouse->z;
         event.motion.pressure = mouse->pressure;
         event.motion.xrel = xrel;
         event.motion.yrel = yrel;