Add a mechanism of frame for XPath evaluation when entering a function
or a scoped evaluation, also fix a potential problem in predicate
evaluation.

The count was incremented before the allocation
and not fixed in case of failure
* xpath.c: corrects a few instances where the available count of some
           structure is updated before we know the allocation actually
           succeeds

If the structured callback channel is initialized, do not perform
unneeded initialization of the old callback channel to avoid
clobbering of the structured callback channel's data.

if we initialize the structured channel from the sax handler we should also
pass the userData

In case the domain is XML_FROM_VALID, ctxt and schannel are already initialized
earlier with the same data

if error code is XML_ERR_OK return immediately

Conflicts:
	doc/devhelp/Makefile.am