Exit cleanly if malloc() fails to allocate a buffer for a copy of the

current MBR.

PR:		205322
Submitted by:	Alexander Kuleshov <kuleshovmail@gmail.com>
MFC after:	1 week

usr.sbin/boot0cfg/boot0cfg.c

@@ -337,6 +337,8 @@ read_mbr(const char *disk, u_int8_t **mbr, int check_version)
 	return (mbr_size);
     }
     *mbr = malloc(sizeof(buf));
+    if (mbr == NULL)
+	errx(1, "%s: unable to allocate MBR buffer", disk);
     memcpy(*mbr, buf, sizeof(buf));
     close(fd);