fix crash on windows with nullptr in operator<<

E.g. http://crashreport.libreoffice.org/stats/crash_details/8962b764-6550-45c9-b08b-e99f5c968d61

Change-Id: I1153c0ab35266006ada10611e79b770ef1c0cd4f
Reviewed-on: https://gerrit.libreoffice.org/39413Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>

external/liborcus/UnpackedTarball_liborcus.mk

@@ -18,6 +18,7 @@ $(eval $(call gb_UnpackedTarball_add_patches,liborcus,\
 	external/liborcus/rpath.patch.0 \
 	external/liborcus/visibility.patch.0 \
 	external/liborcus/iOS.patch \
+	external/liborcus/nullptr-in-ostringstream.patch.1 \
 ))
 
 ifeq ($(OS),WNT)

external/liborcus/nullptr-in-ostringstream.patch.1

+diff -ur liborcus.org/src/liborcus/xml_context_base.cpp liborcus/src/liborcus/xml_context_base.cpp
+--- liborcus.org/src/liborcus/xml_context_base.cpp	2017-06-30 14:32:19.044195127 +0200
++++ liborcus/src/liborcus/xml_context_base.cpp	2017-06-30 14:34:27.580268421 +0200
+@@ -171,7 +171,7 @@
+ 
+     // Create a generic error message.
+     ostringstream os;
+-    os << "element '" << ns << ":" << m_tokens.get_token_name(name) << "' expected, but '";
++    os << "element '" << (ns ? ns : "") << ":" << m_tokens.get_token_name(name) << "' expected, but '";
+     os << elem.first << ":" << m_tokens.get_token_name(elem.second) << "' encountered.";
+     throw xml_structure_error(os.str());
+ }