gethosts: Return EAI_MEMORY on allocation failure

All other cases of failures due to lack of memory return EAI_MEMORY, so
it seems wrong to return EAI_SYSTEM here.  The only reason
convert_hostent_to_gaih_addrtuple could fail is on calloc failure.

Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
Reviewed-by: DJ Delorie <dj@redhat.com>