[Android] Don't try to display without a surface

* Android apps would still try to call `eglSwapBuffers()` even though
they don't have a valid surface right now (i.e. due to being inactive/in
background).

src/SFML/Window/EglContext.cpp

@@ -180,6 +180,7 @@ bool EglContext::makeCurrent()
 ////////////////////////////////////////////////////////////
 void EglContext::display()
 {
+    if (m_surface != EGL_NO_SURFACE)
         eglCheck(eglSwapBuffers(m_display, m_surface));
 }