Bug 620496 - schema compiler: reject invalid paths

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.

gio/tests/schema-tests/invalid-path.gschema.xml

@@ -0,0 +1,3 @@
+<schemalist>
+  <schema id='invalid-path' path='/app/myapp'/>
+</schemalist>