Update tinyxml2

Author: sbaluja

src/aws-cpp-sdk-core/source/external/tinyxml2/tinyxml2.cpp

@@ -119,6 +119,9 @@ This file has been modified from its original version by Amazon:
     #define TIXML_VSNPRINTF	vsnprintf
     static inline int TIXML_VSCPRINTF( const char* format, va_list va )
     {
+        if (!format) {
+            return 0;
+        }
         int len = vsnprintf( 0, 0, format, va );
         TIXMLASSERT( len >= 0 );
         return len;
@@ -2356,8 +2359,10 @@ static FILE* callfopen( const char* filepath, const char* mode )
 }
 
 void XMLDocument::DeleteNode( XMLNode* node )	{
-    TIXMLASSERT( node );
-    TIXMLASSERT(node->_document == this );
+    if(node == 0) {
+        return; // check for null pointer
+    }
+    TIXMLASSERT(node->_document == this);
     if (node->_parent) {
         node->_parent->DeleteChild( node );
     }