[fib algo][dxr] Fix division by zero.

A division by zero would occur if DXR would be activated on a vnet
with no IP addresses configured on any interfaces.

PR:		257965
MFC after:	3 days
Reported by:	Raul Munoz

(cherry picked from commit eb3148c)

sys/netinet/in_fib_dxr.c

@@ -982,7 +982,9 @@ dxr_build(struct dxr *dxr)
 	FIB_PRINTF(LOG_INFO, da->fd, "D%dR, %d prefixes, %d nhops (max)",
 	    DXR_D, rinfo.num_prefixes, rinfo.num_nhops);
 #endif
-	i = dxr_tot_size * 100 / rinfo.num_prefixes;
+	i = dxr_tot_size * 100;
+	if (rinfo.num_prefixes)
+		i /= rinfo.num_prefixes;
 	FIB_PRINTF(LOG_INFO, da->fd, "%d.%02d KBytes, %d.%02d Bytes/prefix",
 	    dxr_tot_size / 1024, dxr_tot_size * 100 / 1024 % 100,
 	    i / 100, i % 100);