MDEV-19740 Debug build of 10.3.15 FTBFS

* Replace LINT_INIT for non-struct types with ctor initializers;
* Check BUILD_DEPS list is not empty so REMOVE_DUPLICATES won't throw
  error.

sql/sql_explain.cc

@@ -444,6 +444,7 @@ uint Explain_union::make_union_table_name(char *buf)
       break;
     default:
       DBUG_ASSERT(0);
+      type= {NULL, 0};
   }
   memcpy(buf, type.str, (len= (uint)type.length));