Fix bug in strinfo

We can't search for a larger needle inside of a smaller haystack, and
unsigned integer subtraction tends to result in very large numbers
rather than small ones.

Add a check for this case and abort out immediately.

Also add a test case (lifted directly from the docs) that demonstrates
the problem.

Issue discovered and tracked down by Milan Bouchet-Valat

gio/strinfo.c

@@ -147,6 +147,9 @@ strinfo_scan (const guint32 *strinfo,
 {
   guint i = 0;
 
+  if (length < n_words)
+    return -1;
+
   while (i <= length - n_words)
     {
       guint j = 0;