0023403: Crash when parsing an expression with lexical error

Fix double freeing of memory in case when lexical error is detected by parser:
- revert previous fix, instead nullify the global buffer after its freeing in ExprIntrp_stop_string().

Added new QA command CR23403 and test case

src/ExprIntrp/ExprIntrp.lex

@@ -37,6 +37,7 @@ void ExprIntrp_start_string(char* str)
 void ExprIntrp_stop_string()
 {
   ExprIntrp_delete_buffer(ExprIntrp_bufstring);
+  ExprIntrp_bufstring = (YY_BUFFER_STATE) 0;
 }
 
 int yywrap()