MDEV-20425: Fix -Wimplicit-fallthrough

With --skip-debug-assert, DBUG_ASSERT(false) will allow execution to
continue. Hence, we will need /* fall through */ after them.

Some DBUG_ASSERT(0) were replaced by break; when the switch () statement
was followed by DBUG_ASSERT(0).

sql/sql_show.cc

@@ -2569,6 +2569,7 @@ static const LEX_CSTRING *view_algorithm(TABLE_LIST *table)
     return &merge;
   default:
     DBUG_ASSERT(0); // never should happen
+    /* fall through */
   case VIEW_ALGORITHM_UNDEFINED:
     return &undefined;
   }