fix a memory leak that could occur when signing

when signatures were not added because of too many types already
existing at a node, the diff was not being cleaned up; this led to
a memory leak being reported at shutdown.

(cherry picked from commit 2825bdb1ae5be801e7ed603ba2455ed9a308f1f7)

lib/dns/zone.c

@@ -10031,6 +10031,7 @@ cleanup:
 	}
 
 	dns_diff_clear(&_sig_diff);
+	dns_diff_clear(&post_diff);
 
 	for (i = 0; i < nkeys; i++) {
 		dst_key_free(&zone_keys[i]);