OpenGL: remove potential NULL dereference

(cherry picked from commit 267d8be5f35b8a33bf5e634ff5087a8ab9788019)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/video_output/opengl.c

@@ -593,7 +593,7 @@ vout_display_opengl_t *vout_display_opengl_New(video_format_t *fmt,
         vgl->tex_type     = GL_UNSIGNED_SHORT;
     }
     vgl->chroma = vlc_fourcc_GetChromaDescription(vgl->fmt.i_chroma);
-    vgl->use_multitexture = vgl->chroma->plane_count > 1;
+    vgl->use_multitexture = vgl->chroma && vgl->chroma->plane_count > 1;
 
     /* Texture size */
     for (unsigned j = 0; j < vgl->chroma->plane_count; j++) {