Fixed a bug report from Frederic Bouvier:

as a follow-up of my previous message, I found that in panel.cxx, function
const char *FGTextLayer::Chunk::getValue () const, there is the use of a
member variable _buf that seems to be uninitialized.

src/Cockpit/panel.cxx

@@ -1096,6 +1096,7 @@ const char *
 FGTextLayer::Chunk::getValue () const
 {
   if (test()) {
+    _buf[0] = '\0';
     switch (_type) {
     case TEXT:
       sprintf(_buf, _fmt.c_str(), _text.c_str());