Compare EGL surface with EGL_NO_SURFACE for consistency

No real changes because EGL_NO_SURFACE is just 0 anyhow, but write the
check for the surface being valid in a way consistent with all the other
ones in this file.

src/unix/glegl.cpp

@@ -700,7 +700,7 @@ wxGLCanvasEGL::~wxGLCanvasEGL()
     }
 #endif // GDK_WINDOWING_WAYLAND
 
-    if ( m_surface )
+    if ( m_surface != EGL_NO_SURFACE )
         eglDestroySurface(m_display, m_surface);
 #ifdef GDK_WINDOWING_WAYLAND
     DestroyWaylandSubsurface();