chdone(): Prevent returning uninitialized scalar value.

If there is an error different from ERESTART, there is some
chance that we may end up accessing an uninitialized value. This
doesn't seem likely/possible but initialize announce_buf[0],
just in case.

CID:	1006486

sys/cam/scsi/scsi_ch.c

@@ -651,6 +651,7 @@ chdone(struct cam_periph *periph, union ccb *done_ccb)
 		} else {
 			int error;
 
+			announce_buf[0] = '\0';
 			error = cherror(done_ccb, CAM_RETRY_SELTO,
 					SF_RETRY_UA | SF_NO_PRINT);
 			/*