[lmi] xmlwrapp '-Wnull-dereference' warning

[Greg Chicares]

Enabling '-Wnull-dereference' for lmi builds, I found:

In file included from /opt/lmi/src/lmi/xml_xslt_wrapp.cpp:35:0:
/opt/lmi/third_party/src/libxml/node_manip.cxx: In member function 
'xml::node::iterator xml::node::
erase(const xml::node::iterator&)':
/opt/lmi/third_party/src/libxml/node_manip.cxx:140:16: error: potential null 
pointer dereference [-Werror=null-dereference]
     xmlNodePtr after = to_erase->next;
                ^~~~~

This simple patch avoids the warning, although...
 - I'm not sure it's right (I'm guessing that this is a C-language
   linked list inside libxml2, whose end is a null pointer)
 - this may not be stylistically consonant with the rest of xmlwrapp,
   e.g., because '0' is used elsewhere in this file where I've written
   'nullptr'

---------8<--------8<--------8<--------8<--------8<--------8<--------8<-------
diff -U3 /opt/lmi/third_party/src/libxml/node_manip.cxx ./node_manip.cxx
--- /opt/lmi/third_party/src/libxml/node_manip.cxx      2018-03-06 
00:27:08.000000000 +0000
+++ ./node_manip.cxx    2019-03-22 00:06:17.520438989 +0000
@@ -137,7 +137,7 @@
 
 xmlNodePtr xml::impl::node_erase(xmlNodePtr to_erase)
 {
-    xmlNodePtr after = to_erase->next;
+    xmlNodePtr after = to_erase ? to_erase->next : nullptr;
 
     xmlUnlinkNode(to_erase);
     xmlFreeNode(to_erase);
--------->8-------->8-------->8-------->8-------->8-------->8-------->8-------

I also wonder whether a change like this would be desirable:

-#include "node_manip.h"
+#include "libxml/node_manip.h"
or
+#include <libxml/node_manip.h>

because a few lines later I see <libxml/another-header.h>:

// libxml includes
#include <libxml/tree.h>