drm/ast: potential dereference of null pointer

The return value of kzalloc() needs to be checked.
To avoid use of null pointer '&ast_state->base' in case of the
failure of alloc.

Fixes: f0adbc38 ("drm/ast: Allocate initial CRTC state of the correct size")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20211214014126.2211535-1-jiasheng@iscas.ac.cn

drivers/gpu/drm/ast/ast_mode.c

@@ -1121,7 +1121,10 @@ static void ast_crtc_reset(struct drm_crtc *crtc)
 	if (crtc->state)
 		crtc->funcs->atomic_destroy_state(crtc, crtc->state);
 
-	__drm_atomic_helper_crtc_reset(crtc, &ast_state->base);
+	if (ast_state)
+		__drm_atomic_helper_crtc_reset(crtc, &ast_state->base);
+	else
+		__drm_atomic_helper_crtc_reset(crtc, NULL);
 }
 
 static struct drm_crtc_state *