Optimizing divisions with a signed numerator
Minor div optimization -- reducing denominators
Moves opus_packet_get_samples_per_frame() to opus.c
Skips comb filter overlap when the parameters didn't change.
Fixes internal check_control_input() error checking.

The code was wrong, but did not have any impact on user-visible behaviour
because all it did was change an internal-only error code.

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