Fail gracefully when encountering 'in fragmentProcessor' in raster backend

SkSLSlide was triggering this assert - now it prints an error every
frame, but doesn't crash.

Bug skia:9941
Change-Id: I4c02a89c8d824acc71ab595af99e1df2f5fc980d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/272639
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Tyler Denniston <tdenniston@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Auto-Submit: Brian Osman <brianosman@google.com>
1 file changed