Avoid overflow and timeout in SkPathWriter::assemble

I was unable to make a test case that caused an overflow
and didn't timeout, but I think the possibility exists
for both.

This removes that, adds a few defensive asserts, and
makes one assert actually checked at runtime, just to be safe.

Change-Id: I4ce112624cb346ac7597466ac9c78402c5061e1c
Bug: 513973560
Fixed: 513973560
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1239556
Reviewed-by: Thomas Smith <thomsmit@google.com>