Fixes https://github.com/davidgiven/ack/issues/188
One call to n_coerc() omits the 6th and last argument. This worked in
traditional K&R C, but stops working if we declare n_coerc() with a
prototype of all 6 parameters.
Change the last parameter to a pointer. Declare n_coerc() with
prototype, so it now requires all 6 arguments. Pass NULL when have no
iocc_t. This NULL exists only to satisfy the prototype; n_coerc()
will not use this NULL.
A different fix would declare n_coerc() with 5 parameters and `...`,
then use <stdarg.h> to read the 6th argument when it exists.
