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
The GSettings schema compiler was accepting any string as a path. It is
probably quite a common mistake to suspect that '/apps/foo' is a valid
path name when this will cause all sorts of trouble later. Check for
this case and report the error.
