utf8.c: convert assertion from English to C

Coverity (CID 584855) complains that _invlist_search can return negative
values. There is a comment that asserts this cannot happen here. Convert
that comment to a C-level assert(). I don't know if that will silence
Coverity, but it can't hurt to verify the invariant at least in
debugging builds.

(Also, convert RT ticket ID to Github issue number and do the
PERL_ARGS_ASSERT_... thing first.)

Author: mauke

utf8.c

@@ -4143,13 +4143,14 @@ Perl_inverse_folds_(pTHX_ const UV cp, U32 * first_folds_to,
      * constructed with this size (to save space and memory), and we return
      * pointers, so they must be this size */
 
+    PERL_ARGS_ASSERT_INVERSE_FOLDS_;
+
     /* 'index' is guaranteed to be non-negative, as this is an inversion map
-     * that covers all possible inputs.  See [perl #133365] */
+     * that covers all possible inputs.  See [GH #16624] */
     SSize_t index = _invlist_search(PL_utf8_foldclosures, cp);
+    assert(index >= 0);
     I32 base = _Perl_IVCF_invmap[index];
 
-    PERL_ARGS_ASSERT_INVERSE_FOLDS_;
-
     if (base == 0) {            /* No fold */
         *first_folds_to = 0;
         *remaining_folds_to = NULL;