LJ_FR2: Fix slot 1 handling.

Contributed by Peter Cawley.

src/jit/dump.lua

@@ -338,6 +338,8 @@ local function formatk(tr, idx, sn)
   elseif t == 21 then -- int64_t
     s = sub(tostring(k), 1, -3)
     if sub(s, 1, 1) ~= "-" then s = "+"..s end
+  elseif sn == 0x1057fff then -- SNAP(1, SNAP_FRAME | SNAP_NORESTORE, REF_NIL)
+    return "----" -- Special case for LJ_FR2 slot 1.
   else
     s = tostring(k) -- For primitives.
   end