maintenance --unregister: fix uninit'd data use & -Wdeclaration-after-statement

Since (maintenance: add option to register in a specific config,
2022-11-09) we've been unable to build with "DEVELOPER=1" without
"DEVOPTS=no-error", as the added code triggers a
"-Wdeclaration-after-statement" warning.

And worse than that, the data handed to git_configset_clear() is
uninitialized, as can be spotted with e.g.:

	./t7900-maintenance.sh -vixd --run=23 --valgrind
	[...]
	+ git maintenance unregister --force
	Conditional jump or move depends on uninitialised value(s)
	   at 0x6B5F1E: git_configset_clear (config.c:2367)
	   by 0x4BA64E: maintenance_unregister (gc.c:1619)
	   by 0x4BD278: cmd_maintenance (gc.c:2650)
	   by 0x409905: run_builtin (git.c:466)
	   by 0x40A21C: handle_builtin (git.c:721)
	   by 0x40A58E: run_argv (git.c:788)
	   by 0x40AF68: cmd_main (git.c:926)
	   by 0x5D39FE: main (common-main.c:57)
	 Uninitialised value was created by a stack allocation
	   at 0x4BA22C: maintenance_unregister (gc.c:1557)

Let's fix both of these issues, and also move the scope of the
variable to the "if" statement it's used in, to make it obvious where
it's used.

Helped-by: Johannes Schindelin <[email protected]>
Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Taylor Blau <[email protected]>

Author: avar

builtin/gc.c

@@ -1550,11 +1550,13 @@ static int maintenance_unregister(int argc, const char **argv, const char *prefi
 		usage_with_options(builtin_maintenance_unregister_usage,
 				   options);
 
-	struct config_set cs;
 	if (config_file) {
+		struct config_set cs;
+
 		git_configset_init(&cs);
 		git_configset_add_file(&cs, config_file);
 		list = git_configset_get_value_multi(&cs, key);
+		git_configset_clear(&cs);
 	} else {
 		list = git_config_get_value_multi(key);
 	}
@@ -1590,7 +1592,6 @@ static int maintenance_unregister(int argc, const char **argv, const char *prefi
 		die(_("repository '%s' is not registered"), maintpath);
 	}
 
-	git_configset_clear(&cs);
 	free(maintpath);
 	return 0;
 }