Fix incorrect error handling

Originally discovered by Coverity as an unused value and a NULL
dereference, it was actually an unfinished error condition check.

Fixes CID 469486: Code maintainability issues (UNUSED_VALUE)
Fixes CID 469485: Null pointer dereferences (FORWARD_NULL)

Signed-off-by: Simo Sorce <simo@redhat.com>

src/session.c

@@ -559,11 +559,13 @@ CK_RV p11prov_context_specific_login(P11PROV_SESSION *session, P11PROV_URI *uri,
     slot = p11prov_get_slot_by_id(sctx, p11prov_session_slotid(session));
     if (!slot) {
         ret = CKR_GENERAL_ERROR;
+        goto done;
     }
 
     ret =
         token_login(session, uri, pw_cb, pw_cbarg, slot, CKU_CONTEXT_SPECIFIC);
 
+done:
     p11prov_return_slots(sctx);
     return ret;
 }