entry.c: use the right index instead of the_index

checkout-index.c needs update because if checkout->istate is NULL,
ie_match_stat() will crash. Previously this is ie_match_stat(&the_index, ..)
so it will not crash, but it is not technically correct either.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout-index.c

@@ -190,6 +190,7 @@ int cmd_checkout_index(int argc, const char **argv, const char *prefix)
 
 	argc = parse_options(argc, argv, prefix, builtin_checkout_index_options,
 			builtin_checkout_index_usage, 0);
+	state.istate = &the_index;
 	state.force = force;
 	state.quiet = quiet;
 	state.not_new = not_new;