builtin-show-branch.c: guard config parser from value=NULL

showbranch.default configuration expects a string value.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-show-branch.c

@@ -536,6 +536,8 @@ static void append_one_rev(const char *av)
 static int git_show_branch_config(const char *var, const char *value)
 {
 	if (!strcmp(var, "showbranch.default")) {
+		if (!value)
+			return config_error_nonbool(var);
 		if (default_alloc <= default_num + 1) {
 			default_alloc = default_alloc * 3 / 2 + 20;
 			default_arg = xrealloc(default_arg, sizeof *default_arg * default_alloc);