Fix the issue of segmentation fault when the pattern of a mbe is not setted. For Skia graphic engine, a mbe does not always have a pattern. For example, a color paint does not set a pattern on a mbe for a shape. So, we check if pattern is setted before accessing it.

<LEFT>		Turn to left.
<RIGHT>		Turn to right.
<UP>		Turn to up.
<DOWN>		Turn to down.
<SPACE>		To Fire.