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>