toke.c: Avoid some UTF8SKIPs

This value is returnable from the called functions.  Use it instead of
re-deriving it

Author: khwilliamson

toke.c

@@ -2964,14 +2964,15 @@ Perl_get_and_check_backslash_N_name(pTHX_ const char* s,
             s += 2;
         }
         else {
+            Size_t advance;
             if (! _invlist_contains_cp(PL_utf8_charname_begin,
                                        utf8_to_uv_or_die((const U8 *) s,
                                                          (const U8 *) e,
-                                                         NULL)))
+                                                         &advance)))
             {
                 goto bad_charname;
             }
-            s += UTF8SKIP(s);
+            s += advance;
         }
 
         while (s < e) {
@@ -2992,14 +2993,15 @@ Perl_get_and_check_backslash_N_name(pTHX_ const char* s,
                 s += 2;
             }
             else {
+                Size_t advance;
                 if (! _invlist_contains_cp(PL_utf8_charname_continue,
                                            utf8_to_uv_or_die((const U8 *) s,
                                                              (const U8 *) e,
-                                                             NULL)))
+                                                             &advance)))
                 {
                     goto bad_charname;
                 }
-                s += UTF8SKIP(s);
+                s += advance;
             }
         }
     }