translate: generic plc not filled in after translation

If during translation a codec could not handle a given frame the translation
core would return NULL, thus not passing along the "missing" frame. Due to this
there was no frame to apply generic plc to, thus rendering it useless.

This patch makes it so the translation core produces an interpolated slin frame
in the cases where an attempt was made to translate to slin, but failed. This
interpolated frame is then passed along and can be used by the generic plc
algorithms to fill in the frame.

ASTERISK-27814 #close

Change-Id: I133d084da87adef913bf2ecc9c9240e3eaf4f40a