Assert when trying to adjust _start in "needs delete" mode.

tinyxml2.cpp

@@ -150,6 +150,8 @@ char* StrPair::ParseName( char* p )
 
 void StrPair::CollapseWhitespace()
 {
+    // Adjusting _start would cause undefined behavior on delete[]
+    TIXMLASSERT( ( _flags & NEEDS_DELETE ) == 0 );
     // Trim leading space.
     _start = XMLUtil::SkipWhiteSpace( _start );