A to-be-closed variable must have a closable value (or be nil)

It is an error for a to-be-closed variable to have a non-closable
non-nil value when it is being closed. This situation does not seem to
be useful and often hints to an error. (Particularly in the C API, it is
easy to change a to-be-closed index by mistake.)

ldebug.h

@@ -22,6 +22,8 @@
 #define ABSLINEINFO	(-0x80)
 
 LUAI_FUNC int luaG_getfuncline (const Proto *f, int pc);
+LUAI_FUNC const char *luaG_findlocal (lua_State *L, CallInfo *ci, int n,
+                                                    StkId *pos);
 LUAI_FUNC l_noret luaG_typeerror (lua_State *L, const TValue *o,
                                                 const char *opname);
 LUAI_FUNC l_noret luaG_forerror (lua_State *L, const TValue *o,