MythUIFileBrowser: Fix a segfault if the 'back' button is missing.

The 'back' button is optional in that dialog so make sure we have one before
trying to update it.
(cherry picked from commit f1ab9ed)

mythtv/libs/libmyth/mythuifilebrowser.cpp

@@ -499,10 +499,14 @@ void MythUIFileBrowser::updateRemoteFileList()
         item->SetText(QString("0"), "filesize");
         item->SetText(m_parentDir, "fullpath");
         item->DisplayState(type, "nodetype");
-        m_backButton->SetEnabled(true);
+        if (m_backButton)
+            m_backButton->SetEnabled(true);
     }
     else
-        m_backButton->SetEnabled(false);
+    {
+        if (m_backButton)
+            m_backButton->SetEnabled(false);
+    }
 
     QStringList::const_iterator it = slist.begin();
     while (it != slist.end())
@@ -667,7 +671,9 @@ void MythUIFileBrowser::updateLocalFileList()
         }
     }
 
-    m_backButton->SetEnabled(showBackButton);
+    if (m_backButton)
+        m_backButton->SetEnabled(showBackButton);
+
     m_locationEdit->SetText(m_subDirectory);
 }