Update pixmap asserts to identify negative values explicitly

This would have aided in debugging the linked bug.

Change-Id: I930cce52f295d33643ff90eacd313cff0542e9bb
Bug: b/447041289
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1065436
Commit-Queue: Kaylee Lubick <kjlubick@google.com>
Reviewed-by: Florin Malita <fmalita@google.com>
5 files changed