image/draw: fold TestClipWithNilMP into TestClip.

https://go-review.googlesource.com/#/c/1876/ introduced a new
TestClipWithNilMP test, along with a code change that fixed a panic,
but the existing TestClip test already contained almost enough machinery
to cover that bug.

There is a small code change in this CL, but it is a no-op: (*x).y is
equivalent to x.y for a pointer-typed x, but the latter is cleaner.

Change-Id: I79cf6952a4999bc4b91f0a8ec500acb108106e56
Reviewed-on: https://go-review.googlesource.com/2304


Reviewed-by: Dave Cheney <dave@cheney.net>