grep: correctly initialize help-all option

The "help-all" option is being initialized with a wrong value.
While being semantically wrong this can also cause a segmentation
fault in gcc on ARMv7 hardfloat platforms with a hardened
toolchain. Fix this by initializing with a NULL value.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Reviewed-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/grep.c

@@ -740,7 +740,7 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 			PARSE_OPT_OPTARG, NULL, (intptr_t)default_pager },
 		OPT_BOOL(0, "ext-grep", &external_grep_allowed__ignored,
 			 N_("allow calling of grep(1) (ignored by this build)")),
-		{ OPTION_CALLBACK, 0, "help-all", &options, NULL, N_("show usage"),
+		{ OPTION_CALLBACK, 0, "help-all", NULL, NULL, N_("show usage"),
 		  PARSE_OPT_HIDDEN | PARSE_OPT_NOARG, help_callback },
 		OPT_END()
 	};