Linux: Fix bad window error when using the XEmbedComponent

The issue was reproducible by opening a plugin window in the AudioPluginHost.

Author: szarvas

modules/juce_audio_processors/format_types/juce_LV2PluginFormat.cpp

@@ -3190,6 +3190,7 @@ class ConfiguredEditorComponent final : public Component,
             Inner() : XEmbedComponent (true, true)
             {
                 setOpaque (true);
+                setVisible (true);
                 addToDesktop (0);
             }
         };

modules/juce_gui_extra/native/juce_XEmbedComponent_linux.cpp

@@ -459,6 +459,9 @@ class XEmbedComponent::Pimpl  : private ComponentListener
     //==============================================================================
     bool getXEmbedMappedFlag()
     {
+        if (client == 0)
+            return false;
+
         XWindowSystemUtilities::GetXProperty embedInfo (getDisplay(), client, infoAtom, 0, 2, false, infoAtom);
 
         if (embedInfo.success && embedInfo.actualFormat == 32