Null deref coverity finding

[trufae]

________________________________________________________________________________________________________
*** CID [1565744](tel:1565744):  Null pointer dereferences  (FORWARD_NULL)
/shlr/qjs/src/quickjs.c: 40918 in js_iterator_helper_next()
40912             break;
40913         default:
40914             abort();
40915         }
40916     
40917     done:
   CID [1565744](tel:1565744):  Null pointer dereferences  (FORWARD_NULL)
   Dereferencing null pointer "it".
40918         it->done = magic == GEN_MAGIC_NEXT ? *pdone : TRUE;
40919         it->executing = FALSE;
40920         return ret;
40921     fail:
40922         if (it) {
40923             /* close the iterator object, preserving pending exception */

[saghul]

Good catch! Fix in #706