"Fix" some ImageFilter fuzzer issues

"Fix" some ImageFilter fuzzer issues

SkClipOp.h & SkPictureFlat.h
   Invalid SkClipOps were getting through - the question here is where (for a class enum) is a good place to put the k*Mask definition.

SkPath1DPathEffect
   NaNs were getting past.

SkBlurMaskFilter
   Assert wasn't necessary since we whacked the flag on the next line.

Change-Id: I87f95ad39f4760284f881d7c4500eb82fcdba282
Reviewed-on: https://skia-review.googlesource.com/6194
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Herb Derby <herb@google.com>