check for pairs with scm_is_pair, not scm_nimp

check for pairs with scm_is_pair, not scm_nimp

* libguile/array-map.c (scm_ra_matchp, scm_ramapc):
* libguile/dynwind.c (scm_swap_bindings):
* libguile/hooks.c (hook_print, scm_c_run_hook, scm_c_run_hookn):
* libguile/objprop.c (scm_object_property, scm_set_object_property_x):
  Use !scm_is_pair as the termination condition, not scm_imp.