GTK: Fix an uninitialized variable causing potential crashes in the joypad config dialog.

src/gtk/joypadconfig.cpp

@@ -51,7 +51,8 @@ JoypadConfigDialog::JoypadConfigDialog(Config::Section * _poConfig) :
   m_oTable(G_N_ELEMENTS(m_astKeys), 2, false),
   m_bUpdating(false),
   m_ePad(PAD_MAIN),
-  m_poConfig(_poConfig)
+  m_poConfig(_poConfig),
+  m_iCurrentEntry(-1)
 {
   // Joypad selection
   m_oTitleCombo.append_text("1");