[ganesh] Avoid overflow when combining AtlasTextOps

Bug: 512435118
Fixed: 512435118
Change-Id: I9bc2bae54830ed32487c03c33fae7b314b73f209
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1232978
Commit-Queue: Nicolette Prevost <nicolettep@google.com>
Reviewed-by: Nicolette Prevost <nicolettep@google.com>
Auto-Submit: Michael Ludwig <michaelludwig@google.com>