wma: Make bitstream API more robust.

wma: Make bitstream API more robust.

The ->buffer_end field of struct getbit_context is set but never
used. In fact, we never check bounds and happily read beyond the
supplied data buffer.

Fix this by replacing the field by ->num_bits, an integer which
is initialized in init_get_bits() to the number of bits available.
All functions which read the bitstream are modified to check bounds.