Clearer code for controlling maximum registers

Plus, added a test to check that limit.

testes/code.lua

@@ -445,5 +445,20 @@ do   -- string constants
   assert(T.listk(f2)[1] == nil)
 end
 
+
+do   -- check number of available registers
+  -- 1 register for local + 1 for function + 252 arguments
+  local source = "local a; return a(" .. string.rep("a, ", 252) .. "a)"
+  local prog = T.listcode(assert(load(source)))
+  -- maximum valid register is 254
+  for i = 1, 254 do
+    assert(string.find(prog[2 + i], "MOVE%s*" .. i))
+  end
+  -- one more argument would need register #255 (but that is reserved)
+  source = "local a; return a(" .. string.rep("a, ", 253) .. "a)"
+  local _, msg = load(source)
+  assert(string.find(msg, "too many registers"))
+end
+
 print 'OK'