Merge pull request #401 from Dmitry-Me/assertForNonNullString

Assertion in StrPair::SetStr()

tinyxml2.cpp

@@ -177,6 +177,7 @@ void StrPair::Reset()
 
 void StrPair::SetStr( const char* str, int flags )
 {
+    TIXMLASSERT( str );
     Reset();
     size_t len = strlen( str );
     TIXMLASSERT( _start == 0 );