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.

ldebug.h

@@ -53,6 +53,7 @@ LUAI_FUNC l_noret luaG_tointerror (lua_State *L, const TValue *p1,
                                                  const TValue *p2);
 LUAI_FUNC l_noret luaG_ordererror (lua_State *L, const TValue *p1,
                                                  const TValue *p2);
+LUAI_FUNC l_noret luaG_errnnil (lua_State *L, LClosure *cl, int k);
 LUAI_FUNC l_noret luaG_runerror (lua_State *L, const char *fmt, ...);
 LUAI_FUNC const char *luaG_addinfo (lua_State *L, const char *msg,
                                                   TString *src, int line);