Fixing a failure in "mtr --ps xml",

introduced by the patch for MDEV-8466 and MDEV-8468.
Using --enable_prepare_warnings and --disable_prepare_warnings
around the affected query, to have the same warning in regular
and --ps mtr runs.

mysql-test/t/xml.test

@@ -674,7 +674,9 @@ SELECT ExtractValue('<a><b>xxx</c></a>','/a/b');
 --echo # Bug#58175 xml functions read initialized bytes when conversions happen
 --echo #
 SET NAMES latin1;
+--enable_prepare_warnings
 SELECT UPDATEXML(CONVERT('' USING swe7), TRUNCATE('',1), 0);
+--disable_prepare_warnings
 
 --echo #
 --echo # Bug#12375190: UPDATEXML CRASHES ON SIMPLE INPUTS