Fixes an out-of-bounds read issue with the padding handling code

This was reported by Juri Aedla and is limited to reading memory up
to about 60 kB beyond the compressed buffer. This can only be triggered
by a compressed packet more than about 16 MB long, so it's not a problem
for RTP. In theory, it *could* crash an Ogg decoder if the memory just after
the incoming packet is out-of-range.
Fixes a minor CELT->SILK switching glitch in the decoder

By not reinitializing the stereo state during a switch, the old stereo
memory would create a false impulse (i.e. single sample) in the decoded audio.
This change affects the normative part of the decoder. Fortunately,
the modified decoder is still compliant with the specification because
it still easily passes the testvectors. For example, for the float decoder
at 48 kHz, the opus_compare (arbitrary) "quality score" changes from
from 99.9333% to 99.925%.