usnic: gracefully handle when we can't alloc an ACK

The comment didn't match the debugging code (which was ugly, and
apparently never happens, anyway).  Just return and let the sender
retransmit.

opal/mca/btl/usnic/btl_usnic_ack.c

@@ -201,8 +201,7 @@ opal_btl_usnic_ack_send(
     /* Get an ACK frag.  If we don't get one, just discard this ACK. */
     ack = opal_btl_usnic_ack_segment_alloc(module);
     if (OPAL_UNLIKELY(NULL == ack)) {
-        opal_output(0, "====================== No frag for sending the ACK -- skipped");
-        abort();
+        return;
     }
 
     /* send the seq of the lowest item in the window that