gtk: Skip drawing if console surface is NULL

In gtk draw/render callbacks, add an early NULL check for the console
surface and skip drawing if it's NULL. Otherwise, attempting to fetch
its width and height crash. This change fixes Coverity CID 1610328.

In practice, this case wouldn't happen at all because we always install
a placeholder surface to the console when there is nothing to display.

Resolves: Coverity CID 1610328
Signed-off-by: Weifeng Liu <weifeng.liu.z@gmail.com>
Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-ID: <20250714141758.10062-1-weifeng.liu.z@gmail.com>

ui/gtk-egl.c

@@ -72,7 +72,7 @@ void gd_egl_draw(VirtualConsole *vc)
 #endif
     int ww, wh, pw, ph, gs;
 
-    if (!vc->gfx.gls) {
+    if (!vc->gfx.gls || !vc->gfx.ds) {
         return;
     }
 
@@ -112,9 +112,6 @@ void gd_egl_draw(VirtualConsole *vc)
         }
 #endif
     } else {
-        if (!vc->gfx.ds) {
-            return;
-        }
         eglMakeCurrent(qemu_egl_display, vc->gfx.esurface,
                        vc->gfx.esurface, vc->gfx.ectx);
 

ui/gtk-gl-area.c

@@ -48,7 +48,7 @@ void gd_gl_area_draw(VirtualConsole *vc)
     int fbw, fbh;
     int wx_offset, wy_offset;
 
-    if (!vc->gfx.gls) {
+    if (!vc->gfx.gls || !vc->gfx.ds) {
         return;
     }
 
@@ -135,9 +135,6 @@ void gd_gl_area_draw(VirtualConsole *vc)
         }
 #endif
     } else {
-        if (!vc->gfx.ds) {
-            return;
-        }
         gtk_gl_area_make_current(GTK_GL_AREA(vc->gfx.drawing_area));
 
         surface_gl_setup_viewport(vc->gfx.gls, vc->gfx.ds, pw, ph);