Added assert to catch unbalanced DSL FP Start/Ends

Change-Id: Ic2c823ed8d8ac77fd6654cc2db1347eb648edf56
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/371397
Auto-Submit: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
1 file changed