[skottie] Fix empty line indexing issue

It's possible to get empty (count = 0) spans in the line domain map,
with e.g. explicit line breaking ("\r\r\r...").

Related code should guard against this condition to avoid indexing at
invalid offsets.

Bug: b/238046903
Change-Id: I42308f57c680d281b0f8fe020146314146debe2a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/588738
Reviewed-by: Jorge Betancourt <jmbetancourt@google.com>
Commit-Queue: Florin Malita <fmalita@google.com>
1 file changed