Work around overzealous assertion in SkMaskFilter.

Work around overzealous assertion in SkMaskFilter.

When drawing nine-patch filters, we assert that the source contains the
destination. However, at least on the new analytic rect blur path, we can get
0-width destinations, and a 0-width rectangle is considered empty and therefore
*not contained in any other rectangle*, even if the bounds are valid and
numerically properly contained.

This patch makes sure we don't try to draw any of the four corners if they
are of 0 width or height. (The assert doesn't exist on the other codepaths.)

git-svn-id: http://skia.googlecode.com/svn/trunk@8860 2bbb7eff-a529-9590-31e7-b0007b416f81