fix a SkRRect::readFromMemory() fuzzer bug

SkRRect::computeType() can fail to produce an isValid() SkRRect,
with the regression test added here.

We can...
   - land something like this that forces a good rect value,
   - drop the assert,
   - or maybe explore deeper and fix this?

Change-Id: I6d7402a0a4af2141ce45e82d287bfb4806aeb0ec
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/309379
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
2 files changed