'luaL_execresult' does not assume -1 status as error

ISO C is silent about the return of 'system'. Windows sets 'errno' in
case of errors. Linux has several different error cases, with different
return values. ISO C allows 'system' to set 'errno' even if there are no
errors. Here we assume that a status==0 is success (which is the case
on several platforms), otherwise it is an error. If there is an error
number, gives the error based on it. (The worst a spurious 'errno'
can do is to generate a bad error message.) Otherwise uses the normal
results.

liolib.c

@@ -270,6 +270,7 @@ static int io_open (lua_State *L) {
 */
 static int io_pclose (lua_State *L) {
   LStream *p = tolstream(L);
+  errno = 0;
   return luaL_execresult(L, l_pclose(L, p->f));
 }