Fix a crash in skia pipeline, when empty reorder block is first

Fix a crash in skia pipeline, which happens if an empty reorder
barrier is inserted at index 0 in a SkiaDisplayList.
Add a unit test that repro the bug (unit test is crashing wihtout
the fix and passing with the fix).

Test: built and ran skia pipeline with angler-eng and run HWUI unit
tests.

Change-Id: I4aded15021c70cc0ae8daa83bbe8ed0e10a41eef