Fix FP_LOADER_THREAD resource leak

The mechanism of thread termination was changed in 61c2beea and not done
correctly - the thread was not notified that it should terminate.

pcbnew/footprint_preview_panel.cpp

@@ -123,6 +123,15 @@ class FP_THREAD_IFACE
             m_panel = aPanel;
         }
 
+        /**
+         * Get the associated panel.
+         */
+        FOOTPRINT_PREVIEW_PANEL* GetPanel()
+        {
+            MUTLOCK lock( m_lock );
+            return m_panel;
+        }
+
         /**
          * Post an event to the panel, if the panel still exists. Return whether
          * the event was posted.
@@ -217,7 +226,7 @@ public:
 
     virtual void* Entry() override
     {
-        while( !TestDestroy() )
+        while( m_iface->GetPanel() )
         {
             auto ent = m_iface->PopFromQueue();
 
@@ -227,7 +236,7 @@ public:
                 wxMilliSleep( 100 );
         }
 
-        return NULL;
+        return nullptr;
     }
 };