Clearer code for controlling maximum registers

Plus, added a test to check that limit.

lcode.c

@@ -31,10 +31,6 @@
 #include "lvm.h"
 
 
-/* Maximum number of registers in a Lua function (must fit in 8 bits) */
-#define MAXREGS		255
-
-
 #define hasjumps(e)	((e)->t != (e)->f)
 
 
@@ -466,7 +462,7 @@ static int luaK_codek (FuncState *fs, int reg, int k) {
 void luaK_checkstack (FuncState *fs, int n) {
   int newstack = fs->freereg + n;
   if (newstack > fs->f->maxstacksize) {
-    if (newstack >= MAXREGS)
+    if (newstack > MAX_FSTACK)
       luaX_syntaxerror(fs->ls,
         "function or expression needs too many registers");
     fs->f->maxstacksize = cast_byte(newstack);