Fix zero-sized blur with crop rect.

Fix zero-sized blur with crop rect.

[Cherry-pick of 6404981b0c3984ba91b3bda35e60c6c8a8a773c2 to chrome/m49.]

Neither the GPU nor CPU paths were correctly handling the crop rect
in this case.

NOTE: this change adds a new test case to the imageblurcropped GM, so
it will have to be rebaselined.

BUG=skia:4876
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1657773002
NOTREECHECKS=true
NOTRY=true
NOPRESUBMIT=true

Review URL: https://codereview.chromium.org/1663533003 .