SkPath.readFromMemory:  check for error case
SkPath.readFromMemory:  check for error case

Fuzzing can produce inconsistant data, such as flags or bounds.

Also, use std::unique_ptr to reduce the need for calling `delete`.

BUG=skia:6262
BUG=skia:6263
Change-Id: I1de6b5f764cda346bb3cd5cd4698816b6b68f395
Reviewed-on: https://skia-review.googlesource.com/8812
Commit-Queue: Hal Canary <fd9641b23487c8ee78406c27513624c460600766@google.com>
Reviewed-by: Cary Clark <52792aa984ff6c4e9c802c7fa3093ed6a52a791f@google.com>

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