Validate allocation size in GrBufferAllocPool using SkSafeMath.

Bug: 895362

Cherry pick to M71

No-Tree-Checks: true
No-Try: true
No-Presubmit: true
Change-Id: I3a49bcd899760d33789a9df17a0a5f3556c879a5
Reviewed-On: https://skia-review.googlesource.com/c/163485
Reviewed-By: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-on: https://skia-review.googlesource.com/c/170349
Reviewed-by: Brian Salomon <bsalomon@google.com>