Fix NPE for filter with and predicate in path step

[GuntherRademacher]

As reported by Andreas Hengsbach on basex-talk, an NPE can result from

• a path expression, that has
• a context value step, with
• a filter expression, containing
• an and expression, involving
• a function call.

E.g.

import module namespace functx = "http://www.functx.com";

<a/>/.[true() and functx:all-whitespace('')]

This can also be reproduced as simple as

()/.[true() and true()]

It is caused by Path.get rewriting a filter expression to a self step with predicates. In the course of this, the predicates are queried for Flag.POS (via mayBePositional()). But when this is done at parse time, FuncRefs are not resolved yet - yielding an NPE for the flags request.

However it is not necessary to do the rewrite this early, an equivalent rewrite is also done at optimization time, in Filter.optimize. So the fix is to just remove it from Path.get.

[ChristianGruen]

…merci