Fixed wrong error message in 'return math.seed(0)'

Bug introduced in commit 28d829c8: OP_TAILCALL might raise an
error without saving 'pc'. (This commit also fixes a detail in
'testes/uf8.lua'.)

testes/utf8.lua

@@ -66,7 +66,7 @@ local function check (s, t, nonstrict)
    assert(utf8.len(s, pi, pi1 - 1, nonstrict) == 1)
    assert(utf8.len(s, pi, -1, nonstrict) == l - i + 1)
    assert(utf8.len(s, pi1, -1, nonstrict) == l - i)
-   assert(utf8.len(s, 1, pi, -1, nonstrict) == i)
+   assert(utf8.len(s, 1, pi, nonstrict) == i)
   end
 
   local i = 0