dns_rdataslab_merge(): use dns_rdata_compare() instead of compare_rdata()

compare_rdata() was meant to be used as a qsort() callback.  Meanwhile,
dns_rdataslab_merge() calls compare_rdata() for a pair of dns_rdata_t
structures rather than a pair of struct xrdata structures, which is
harmless, but triggers an ubsan warning:

    rdataslab.c:84:33: runtime error: member access within address <address> with insufficient space for an object of type 'const struct xrdata'

Use dns_rdata_compare() instead of compare_rdata() to prevent the
warning from being triggered.

lib/dns/rdataslab.c

@@ -796,7 +796,8 @@ dns_rdataslab_merge(unsigned char *oslab, unsigned char *nslab,
 		else if (nadded == ncount)
 			fromold = ISC_TRUE;
 		else
-			fromold = ISC_TF(compare_rdata(&ordata, &nrdata) < 0);
+			fromold = ISC_TF(dns_rdata_compare(&ordata,
+							   &nrdata) < 0);
 		if (fromold) {
 #if DNS_RDATASET_FIXED
 			offsettable[oorder] = tcurrent - offsetbase;