XPath: Make sure step_push is called with valid nodes

Some steps relied on step_push rejecting null inputs; this is no longer
the case. Additionally stepping now more rigorously filters null inputs.

git-svn-id: https://pugixml.googlecode.com/svn/trunk@1069 99668b35-9821-0410-8761-19e4c4f06640
tests: Assert on out-of-memory in tests

This should never happen but can improve debugging experience for
work-in-progress changes since that avoids memcpy() into negative memory
space (debugger can't backtrace from failed memcpy since it does not set
up the stack frame).

git-svn-id: https://pugixml.googlecode.com/svn/trunk@1070 99668b35-9821-0410-8761-19e4c4f06640