lib-sieve: vacation extension: Enforce subject header limit in Unicod…

…e codepoints rather than bytes.

src/lib-sieve/plugins/vacation/cmd-vacation.c

@@ -1006,7 +1006,7 @@ static int act_vacation_send
 		subject = ctx->subject;
 	}
 
-	subject = str_sanitize(subject, 256);
+	subject = str_sanitize_utf8(subject, 256);
 
 	/* Obtain full To address for reply */