Fix 40/60ms zero-length frame decode failure

Decoding failed with OPUS_BAD_ARG on a packet containing a 40ms or
60ms zero-length frame when it followed a hybrid or MDCT frame.
It now invokes the PLC for the duration of the packet as expected.

Signed-off-by: Jean-Marc Valin <jmvalin@jmvalin.ca>
1 file changed