Fix Error When Input Buffer is Full

In cases where the input buffer is exhausted and the picture has still
not been decoded fully, the multi-thread decoder would read beyond the
allocated size.

Bug: 38328132
Test: asan-enabled POC before/after patch on master.
Change-Id: I88afb150f064a13c44b003d819ac948368b11d10
Merged-In: I8a73b819558407c130dcafeeab1a9bff77688dfc