Silence gcc-10-fanalyzer false positive

If 'state == ft_ordinary' then 'label' can't be NULL. Add
INSIST to reflect this.

(cherry picked from commit 29f1c1e677)

lib/dns/name.c

@@ -1280,6 +1280,7 @@ dns_name_fromtext(dns_name_t *name, isc_buffer_t *source,
 			return (ISC_R_UNEXPECTEDEND);
 		if (state == ft_ordinary) {
 			INSIST(count != 0);
+			INSIST(label != NULL);
 			*label = count;
 			labels++;
 			INSIST(labels <= 127);