Global initialization checks name conflict

Initialization "global a = 10" raises an error if global 'a' is already
defined, that is, it has a non-nil value.

lopcodes.c

@@ -103,6 +103,7 @@ LUAI_DDEF const lu_byte luaP_opmodes[NUM_OPCODES] = {
  ,opmode(0, 0, 0, 0, 1, iABx)		/* OP_CLOSURE */
  ,opmode(0, 1, 0, 0, 1, iABC)		/* OP_VARARG */
  ,opmode(0, 0, 0, 0, 1, iABC)		/* OP_GETVARG */
+ ,opmode(0, 0, 0, 0, 0, iABx)		/* OP_ERRNNIL */
  ,opmode(0, 0, 1, 0, 1, iABC)		/* OP_VARARGPREP */
  ,opmode(0, 0, 0, 0, 0, iAx)		/* OP_EXTRAARG */
 };