iolog: add va_end on error

iolog: add va_end on error

Call va_end when we encounter an error trying to print sample fields.

This was reported by Coverity:

** CID 509197:  API usage errors  (VARARGS)
/iolog.c: 1025 in print_sample_fields()
________________________________________________________________________________________________________
*** CID 509197:  API usage errors  (VARARGS)
/iolog.c: 1025 in print_sample_fields()
1019     	int ret;
1020
1021     	va_start(ap, fmt);
1022     	ret = vsnprintf(*p, *left, fmt, ap);
1023     	if (ret < 0 || ret >= *left) {
1024     		log_err("sample file write failed: %d\n", ret);
>>>     CID 509197:  API usage errors  (VARARGS)
>>>     "va_end" was not called for "ap".
1025     		return -1;
1026     	}
1027     	va_end(ap);
1028
1029     	*p += ret;
1030     	*left -= ret;

Fixes: 3ec6b6da ("iolog: refactor flush_samples()")
Signed-off-by: Vincent Fu <vincent.fu@samsung.com>