BUG/MINOR: dns: ignore trailing dot

As per issue #435 a hostname with a trailing dot confuses our DNS code,
as for a zero length DNS label we emit a null-byte. This change makes us
ignore the zero length label instead.

Must be backported to 1.8.

src/dns.c

@@ -1450,6 +1450,12 @@ int dns_str_to_dn_label(const char *str, int str_len, char *dn, int dn_len)
 			if (i == offset)
 				return -1;
 
+			/* ignore trailing dot */
+			if (i + 2 == str_len) {
+				i++;
+				break;
+			}
+
 			dn[offset] = (i - offset);
 			offset = i+1;
 			continue;