lib/calib_yacc.y: fix segfault in comedi_parse_calibration_file()
lib/calib_yacc.y: fix segfault in comedi_parse_calibration_file()

Don't call fill_inverse_linear_polynomials(priv.parsed_file) if
priv.parsed_file is NULL as that would dereference the NULL pointer
leading to a segmentation fault.

olsonse/comedilib,Linux-Comedi/comedilib,Linux-Comedi/comedilib,olsonse/comedilib,Linux-Comedi/comedilib,Linux-Comedi/comedilib,olsonse/comedilib,Linux-Comedi/comedilib,Linux-Comedi/comedilib,olsonse/comedilib,olsonse/comedilib,olsonse/comedilib