lib: trusty: Assert that at most one handle gets deleted

`_hset_add_item` ensures that we cannot insert duplicates. This change
asserts if `_hset_del_item` encounters duplicates.

Test: build & run Trusty unit tests
Bug: 382291660
Change-Id: I6d8375be46602d0b770acb6dcf99bf21aa16c46f

lib/trusty/uctx.c

diff --git a/lib/trusty/uctx.c b/lib/trusty/uctx.c
index b3a775f..abd6ed1 100644
--- a/lib/trusty/uctx.c
+++ b/lib/trusty/uctx.c
@@ -732,6 +732,7 @@
             free(ref);
         }
     }
+    ASSERT(del_cnt <= 1);
     return del_cnt ? NO_ERROR : ERR_NOT_FOUND;
 }
 
@@ -751,6 +752,7 @@
             handle_set_update_ref(ref, emask, cookie);
         }
     }
+    ASSERT(mod_cnt <= 1);
     return mod_cnt ? NO_ERROR : ERR_NOT_FOUND;
 }