LoadCoreDialog: only try reading from existing corefile

- currently gdb is called on every change of the core file, i.e. on
  every keypress when typing a path. Changed to only do that when the
  file exists

Change-Id: I27f1c21b2561420d72f273244dc6a1bb5fb8bfe7
Reviewed-by: hjk <hjk121@nokiamail.com>

src/plugins/debugger/loadcoredialog.cpp

@@ -363,7 +363,7 @@ bool AttachCoreDialog::useLocalCoreFile() const
 
 void AttachCoreDialog::coreFileChanged(const QString &core)
 {
-    if (!Utils::HostOsInfo::isWindowsHost()) {
+    if (!Utils::HostOsInfo::isWindowsHost() && QFileInfo::exists(core)) {
         Kit *k = d->kitChooser->currentKit();
         QTC_ASSERT(k, return);
         FileName cmd = DebuggerKitInformation::debuggerCommand(k);