debugger: do not write the 'all' (-1) threadspec to the session file

This confuses 2.1 which uses 0 as 'all' value.

src/plugins/debugger/breakhandler.cpp

@@ -265,7 +265,7 @@ void BreakHandler::saveBreakpoints()
             map.insert(_("condition"), data.condition);
         if (data.ignoreCount)
             map.insert(_("ignorecount"), data.ignoreCount);
-        if (data.threadSpec)
+        if (data.threadSpec >= 0)
             map.insert(_("threadspec"), data.threadSpec);
         if (!data.enabled)
             map.insert(_("disabled"), _("1"));