BUG/MEDIUM: arg: empty args list must be dropped

Before commit 80b53ffb1 ("MEDIUM: arg: make make_arg_list() stop after
its own arguments"), consumers of arguments would measure the length of
the string between the first opening and closing parenthesis before
calling make_arg_list(), and this latter one would detect an empty string
early by len==0 and would not allocate an argument list.

Since that commit, this has a changed a bit because the argument parser
is now the one in charge for delimiting the argument string, so the early
test cannot be used anymore. But the argument list is still allocated,
and despite the number of arguments being returned, consumers do not
necessarily rely on it but instead they rely on the non-null arg_p
pointer that used to be allocated only if at least one argument was
present. But as it's now always allocated, the first argument always
carries the first argument's type with an empty value, which confuses
all functions that take a unique optional argument (such as uuid()).

The proper long term solution would be to always use the returned argument
count, but at least we can make sure the function always returns an empty
argument list when fed with an empty set of parenthesis, as it always used
to do. This is what this patch does.

This fix must be backported to 2.2 and fixes github issue #763. Thanks to
Luke Seelenbinder for reporting the problem.

src/arg.c

@@ -395,6 +395,14 @@ int make_arg_list(const char *in, int len, uint64_t mask, struct arg **argp,
 	return -1;
 
  empty_err:
+	/* If we've only got an empty set of parenthesis with nothing
+	 * in between, there is no arg at all.
+	 */
+	if (!pos) {
+		free(*argp);
+		*argp = NULL;
+	}
+
 	if (pos >= min_arg)
 		goto end_parse;