REST: only update bugnote timestamp when changed

Prior to this, any bugnote provided in the Issue PATCH request's body
would see its timestamp updated, even if there was no actual change to
the bugnote's data.

Fixes #33402, PR #1952

Signed-off-by: Damien Regad <dregad@mantisbt.org>

api/soap/mc_issue_api.php

@@ -1156,6 +1156,7 @@ function mc_issue_update( $p_username, $p_password, $p_issue_id, stdClass $p_iss
 
 				if( array_key_exists( $t_bugnote_id, $t_bugnotes_by_id ) ) {
 					$t_bugnote_changed = false;
+					$t_bugnote = $t_bugnotes_by_id[$t_bugnote_id];
 
 					if( $t_bugnote->note !== $t_note['text'] ) {
 						bugnote_set_text( $t_bugnote_id, $t_note['text'] );