Fix dyn:map with namespace nodes

exsltDynMapFunction didn't handle namespace nodes correctly. Namespace
nodes are actually an xmlNs, not an xmlNode and must be special-cased.

The old code initialized the doc pointer in the XPath context struct
with a value read from past the end of the xmlNs struct. Typically,
this resulted in a segfault.

Found with afl-fuzz and ASan.