debugger: fix chain of fallbacks for disassembler view.

Use non-mixed mode if source is not available.

src/plugins/debugger/stackframe.h

@@ -38,16 +38,16 @@ namespace Internal {
 
 struct StackFrame
 {
-    StackFrame(int level = 0);    
+    StackFrame();
     bool isUsable() const;
     QString toToolTip() const;
     QString toString() const;
 
     int level;
     QString function;
-    QString file;  // we try to put an absolute file name in there
-    QString from;
-    QString to;
+    QString file;  // We try to put an absolute file name in there.
+    QString from;  // Sometimes something like "/usr/lib/libstdc++.so.6"
+    QString to;    // Used in ScriptEngine only.
     int line;
     QString address;
 };