container/patricia: mowgli_patricia_elem_find(): avoid potential UB

Identified by Clang v9 UBSAN (-fsanitize=undefined):

    patricia.c:671:17: runtime error: member access within null
                       pointer of type 'union patricia_elem'

src/libmowgli/container/patricia.c

@@ -668,6 +668,9 @@ mowgli_patricia_elem_find(mowgli_patricia_t *dict, const char *key)
 	if (ckey_buf != NULL)
 		mowgli_free(ckey_buf);
 
+	if (delem == NULL)
+		return NULL;
+
 	return &delem->leaf;
 }