Guard against null tops and bottoms

The intersect code is going to dereference fTop and
fBottom of both the left and right. It is a logical
error if they don't exist. Guard against that.

Bug: oss-fuzz:61271
Change-Id: I624e70f9b6b47270a03754061021dc8ce80b922a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/771705
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
1 file changed