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>

src/core/SkDescriptor.cpp

diff --git a/src/core/SkDescriptor.cpp b/src/core/SkDescriptor.cpp
index 7ee966d..941a0d3 100644
--- a/src/core/SkDescriptor.cpp
+++ b/src/core/SkDescriptor.cpp
@@ -178,8 +178,11 @@
     if (!buffer.readPad32(&descriptorHeader, sizeof(SkDescriptor))) { return {}; }
 
     // Basic bounds check on header length to make sure that bodyLength calculation does not
-    // underflow.
-    if (descriptorHeader.getLength() < sizeof(SkDescriptor)) { return {}; }
+    // underflow. Also ensure alignment.
+    if (descriptorHeader.getLength() < sizeof(SkDescriptor) ||
+        !SkIsAlign4(descriptorHeader.getLength())) {
+        return {};
+    }
     uint32_t bodyLength = descriptorHeader.getLength() - sizeof(SkDescriptor);
 
     // Make sure the fLength makes sense with respect to the incoming data.