Improve error handling of libxml2 calls in xml.c

 Improve error handling of libxml2 calls in xml.c
 
 This commit fixes some defects in the backend's xml.c, found upon
 inspection of the internals of libxml2:
 - xmlEncodeSpecialChars() can fail on malloc(), returning NULL back to
 the caller.  xmltext() assumed that this could never happen.  Like other
 code paths, a TRY/CATCH block is added there, covering also the fact
 that cstring_to_text_with_len() could fail a memory allocation, where
 the backend would miss to free the buffer allocated by
 xmlEncodeSpecialChars().
 - Some libxml2 routines called in xmlelement() can return NULL, like
 xmlAddChildList() or xmlTextWriterStartElement().  Dedicated errors are
 added for them.
 - xml_xmlnodetoxmltype() missed that xmlXPathCastNodeToString() can fail
 on an allocation failure.  In this case, the call can just be moved to
 the existing TRY/CATCH block.
 
 All these code paths would cause the server to crash.  As this is
 unlikely a problem in practice, no backpatch is done.  Jim and I have
 caught these defects, not sure who has scored the most.  The contrib
 module xml2/ has similar defects, which will be addressed in a separate
 change.
 
 Reported-by: Jim Jones <jim.jones@uni-muenster.de>
 Reviewed-by: Jim Jones <jim.jones@uni-muenster.de>
 Discussion: https://postgr.es/m/aEEingzOta_S_Nu7@paquier.xyz