scsi: osst: silence underflow warning in osst_verify_frame()

The code looks like this:

	i = ntohl(aux->filemark_cnt);
	if (STp->header_cache != NULL && i < OS_FM_TAB_MAX && (i > STp->filemark_cnt ||
	    STp->first_frame_position - 1 != ntohl(STp->header_cache->dat_fm_tab.fm_tab_ent[i]))) {

If i is negative then it's less than OS_FM_TAB_MAX so we read before
the start of the STp->header_cache->dat_fm_tab.fm_tab_ent[] array.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>