fix: uniqstr are already pointers

* src/uniqstr.c (uniqstr_assert): Remove incorrect double indirection,
and now useless cast.

src/uniqstr.c

@@ -77,8 +77,8 @@ uniqstr_vsprintf (char const *format, ...)
 void
 uniqstr_assert (char const *str)
 {
-  uniqstr *s = hash_lookup (uniqstrs_table, str);
-  if (!s || s != (uniqstr *)str)
+  uniqstr s = hash_lookup (uniqstrs_table, str);
+  if (!s || s != str)
     {
       error (0, 0,
              "not a uniqstr: %s", quotearg (str));