Update ieee488.c
Update ieee488.c

*TST? should return 0 for non-error response.
koeart/scpi-parser,j123b567/scpi-parser,WCP52/gpha,WCP52/gpha,j123b567/scpi-parser,WCP52/gpha,WCP52/gpha,koeart/scpi-parser,RedPitaya/scpi-parser,RedPitaya/scpi-parser