res_mkquery: error out on consecutive final dots in name

the main loop already errors out on zero-length labels within the
name, but terminates before having a chance to check for an erroneous
final zero-length label, instead producing a malformed query packet
with a '.' byte instead of the terminating zero.

rather than poke at the look logic, simply detect this condition early
and error out without doing anything.

this also fixes behavior of getaddrinfo when "." appears in the search
domain list, which produces a name ending in ".." after concatenation,
at least in the sense of no longer emitting malformed packets on the
network. however, due to other issues, the lookup will still fail.

src/network/res_mkquery.c

@@ -13,6 +13,7 @@ int __res_mkquery(int op, const char *dname, int class, int type,
 	int n;
 
 	if (l && dname[l-1]=='.') l--;
+	if (l && dname[l-1]=='.') return -1;
 	n = 17+l+!!l;
 	if (l>253 || buflen<n || op>15u || class>255u || type>255u)
 		return -1;