install: fail early if specifier expansion failed

Before:

systemd[1]: Assertion 'path' failed at src/shared/install.c:288, function install_changes_add(). Aborting.
systemd[1]: Caught <ABRT> from our own process.
systemd[1]: Caught <ABRT>, dumped core as pid 2525.
systemd[1]: Freezing execution

After:

Failed to enable unit: Invalid specifier in user-%J.service

Fixes #26467.

Follow-up for: f5a0162

src/shared/install.c

@@ -1995,6 +1995,8 @@ static int install_info_symlink_wants(
                         install_changes_add(changes, n_changes, q, *s, NULL);
                         if (r >= 0)
                                 r = q;
+
+                        continue;
                 }
 
                 if (!unit_name_is_valid(dst, valid_dst_type)) {