egl_dri2 seems to handle this fine, but egl_glx tries to dereference it in this function:

static int
GLX_egl_config_index(_EGLConfig *conf)
{
   struct GLX_egl_config *GLX_conf = GLX_egl_config(conf);
   return GLX_conf->index;
}

So it segfaults when it tries to create a context on this line in GLX_eglCreateContext:

      GLX_ctx->context =
         glXCreateNewContext(GLX_dpy->dpy,
                             GLX_dpy->fbconfigs[GLX_egl_config_index(conf)],
                             GLX_RGBA_TYPE,
                             GLX_ctx_shared ? GLX_ctx_shared->context : NULL,
                             GL_TRUE);