GN_Flc, Close filter

IN:
     IX = filter handle

OUT, if call successful:
     Fc = 0
     IX = 0
     BC = number of input characters written to filter
     (BC = 0 provided at least one character has been read)
     DE = number of characters read from filter

OUT, if call failed:
     Fc = 1
     A = error code:
          RC_HAND ($08), IX(in) was not a valid filter handle

Registers changed after return:
     ......HL/..IY same
     AFBCDE../IX.. different