Prevent SEGV when resizing with GFX

The xrdp_enc_data contains a union for handling surface commands
and gfx commands. Memory processing is different for these two
options.

The default destructor for the encoder FIFO only knows about surface
commands. Consequently, if the encoder has queued GFX data when the
encoder is closed, the destructor processes the queued data as if
it contained surface commands rather than GFX commands. This typically
causes a SEGV as the drects field of the overlaid surface command
structure is not pointing at anything valid when it is freed.

(cherry picked from commit 809df89)

xrdp/xrdp_encoder.c

@@ -82,8 +82,15 @@ static void
 xrdp_enc_data_destructor(void *item, void *closure)
 {
     XRDP_ENC_DATA *enc = (XRDP_ENC_DATA *)item;
-    g_free(enc->u.sc.drects);
-    g_free(enc->u.sc.crects);
+    if (ENC_IS_BIT_SET(enc->flags, ENC_FLAGS_GFX_BIT))
+    {
+        g_free(enc->u.gfx.cmd);
+    }
+    else
+    {
+        g_free(enc->u.sc.drects);
+        g_free(enc->u.sc.crects);
+    }
     g_free(enc);
 }