tal/path: handle weird case of path_join("")

It seems most sensible to make it a noop, but it definitely shouldn't
access out of bounds as it does.

Reported-by: Russ Dill
Fixes: #61
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/tal/path/path.c

@@ -52,7 +52,7 @@ char *path_join(const tal_t *ctx, const char *base, const char *a)
 	ret = tal_dup_arr(ctx, char, base, len, 1 + strlen(a) + 1);
 	if (!ret)
 		goto out;
-	if (ret[len-1] != PATH_SEP)
+	if (len != 0 && ret[len-1] != PATH_SEP)
 		ret[len++] = PATH_SEP;
 	strcpy(ret + len, a);
 

ccan/tal/path/test/run-join.c

@@ -6,7 +6,7 @@ int main(void)
 {
 	char *path, *ctx = tal_strdup(NULL, "ctx");
 
-	plan_tests(34);
+	plan_tests(36);
 
 	path = path_join(ctx, "foo", "bar");
 	ok1(streq(path, "foo/bar"));
@@ -85,6 +85,11 @@ int main(void)
 	ok1(!path);
 	ok1(!tal_first(ctx));
 
+	path = path_join(ctx, "", "bar");
+	ok1(streq(path, "bar"));
+	ok1(tal_parent(path) == ctx);
+	tal_free(path);
+
 	tal_free(ctx);
 
 	return exit_status();