Bad Canonicalization in mtd

[NSoiffer]

The following causes pattern matching misses because after canonicalization, the mtd is empty:

<math>
	<mtable>
		<mtr>
			<mtd>
				<mstyle scriptlevel="0">
					<mspace width="2em"/>
				</mstyle>
				<mstyle scriptlevel="0">
					<mspace width="1em"/>
				</mstyle>
			</mtd>
		</mtr>
	</mtable>
</math>

Both <mstyle>s need to be present to trigger the bug.