usnic: make the releasing safer

Since the usnic BTL is single-threaded in this area, there really is
no danger, but don't use one of the pointers hanging off the frag
after we return it to the freelist.  Instead, save the endpoint
pointer before returning the frag.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>

opal/mca/btl/usnic/btl_usnic_send.c

@@ -60,13 +60,18 @@ opal_btl_usnic_frag_send_complete(opal_btl_usnic_module_t *module,
     --frag->sf_seg_post_cnt;
 
     /* checks for returnability made inside */
+    opal_btl_usnic_endpoint_t *ep = frag->sf_endpoint;
     opal_btl_usnic_send_frag_return_cond(module, frag);
 
+    // In a short frag segment, the sseg is embedded in the frag.  So
+    // there's no need to return the sseg (because we already returned
+    // the frag).
+
     /* do bookkeeping */
-    ++frag->sf_endpoint->endpoint_send_credits;
+    ++ep->endpoint_send_credits;
 
     /* see if this endpoint needs to be made ready-to-send */
-    opal_btl_usnic_check_rts(frag->sf_endpoint);
+    opal_btl_usnic_check_rts(ep);
 
     ++module->mod_channels[sseg->ss_channel].credits;
 }