avcodec/g723_1enc: Avoid skip_put_bits()

avcodec/g723_1enc: Avoid skip_put_bits()

If a bit is reserved, it matters very much what value it has, because
otherwise a decoder conforming to a future version of the standard might
interpret the output file in an unintended manner. This implies that
one must not use skip_put_bits() for it (which does not give any
guarantees wrt what ends up in the output (in case of a little-endian
bitstream writer (as here) it writes a 0 bit)); given that the reference
encoder as well as the earlier code write a zero bit at this place, the
new code does, too.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>