Fixed DSLParser assertion error uncovered by fuzzer

Bug: oss-fuzz:38108
Change-Id: I0e055d837923f00b982bc395dbf29b6ff59a3b21
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/448896
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
9 files changed