video: fbdev: Fix potential NULL pointer dereference

There is a potential NULL pointer dereference in case
fb_create_modedb() fails and returns NULL.

Signed-off-by: YueHaibing <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Rob Herring <[e-mail]>
Signed-off-by: Bartlomiej Zolnierkiewicz <[e-mail]>

drivers/video/fbdev/core/fbmon.c

···
 	get_monspecs(edid, specs);
 
 	specs->modedb = fb_create_modedb(edid, &specs->modedb_len, specs);
+	if (!specs->modedb)
+		return;
 
 	/*
 	 * Workaround for buggy EDIDs that sets that the first