Validate SkDescriptor length alignment on read

When deserializing an SkDescriptor in MakeFromBuffer, we
must validate that the length is a multiple of 4
(SkIsAlign4). Otherwise, passing an unaligned length to
SkAutoDescriptor triggers an assertion failure/abort
inside SkDescriptor::Alloc.

Bug: b/524792615
Test: skia_unittests --gtest_filter="Descriptor*"
Change-Id: Id3db0fb85c8baa50db617ff93bf34f1d6be0a471
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1287596
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Tzarial <zork@google.com>