aft.c: Fix com_add().

aft.c: Fix com_add().

callbacks shall return zero or negative, if no result is provided. So let
com_ls_callback() return zero if no match was found, or if the audio file table
does not contain any entries. Check for ret > 0 in com_add() instead of ret >=
0, i.e. don't use the result pointer if ret is zero.