show correct exceptions: fix exception memory access bug

src/exceptions.hxx

@@ -16,7 +16,8 @@ class Exception: public std::exception {
     Exception(QString w): _what(w) {}
     ~Exception() throw() {}
     const char* what() const throw() {
-      return _what.toStdString().c_str();
+      _bytes = _what.toUtf8();
+      return _bytes.data();
     }
     void line(int linenr) {
       if (linenr>0) _what=QString::number(linenr)+" "+_what;
@@ -25,6 +26,7 @@ class Exception: public std::exception {
       if (filename.size()) _what=filename+":"+_what;
     }
   protected:
+    QByteArray _bytes;
     QString _what;
 };