Don't stash the cabac state into the SBitStringAux buffer

This fixes crashes on some platforms, and fixes valgrind
errors.
Remove the pBuffer parameter from the entropy encoder stash functions

The parameter didn't make sense since it only pointed to the
pSliceBsa field, which isn't what should be used to stash
the cabac context.
Merge pull request #1467 from mstorsjo/cabac-stash-fix

Don't stash the cabac state into the SBitStringAux buffer