There's a bitstream conformance requirement that says that any block
must subsample to a valid block size with the current subsampling
mode. For example, this means that BLOCK_4X8 is illegal if there is
subsampling in only the horizontal direction (since there is no
BLOCK_2X8).

This patch checks the bitstream is conformant as it reads partition
information in decodeframe.c

BUG=aomedia:875

Change-Id: I18139aa76d6f965282402edbb0b68959478a46c3