Fixe crash on empty directory node.

When there's a directory entry that doesn't contain any children, the lib crashes:

	node_modules\ole-doc\lib\ole-doc.js:161
	      if ( visitEntry.left !== DirectoryTree.Leaf ) {
	                     ^
	TypeError: Cannot read property 'left' of undefined
	    at visit (node_modules\ole-doc\lib\ole-doc.js:161:22)
	    at DirectoryTree._getChildIds (node_modules\ole-doc\lib\ole-doc.js:174:4)
	    at DirectoryTree._buildHierarchy (node_modules\ole-doc\lib\ole-doc.js:135:24)
	    at DirectoryTree._buildHierarchy (node_modules\ole-doc\lib\ole-doc.js:152:12)
	    at Function._.each._.forEach (node_modules\ole-doc\node_modules\underscore\underscore.js:86:24)
	    at DirectoryTree._buildHierarchy (node_modules\ole-doc\lib\ole-doc.js:151:6)
	    at DirectoryTree.load (node_modules\ole-doc\lib\ole-doc.js:127:12)
	    at OleCompoundDoc._readSectors (node_modules\ole-doc\lib\ole-doc.js:410:10)
	    at Object.async.whilst (node_modules\ole-doc\node_modules\async\lib\async.js:562:13)
	    at async.whilst (node_modules\ole-doc\node_modules\async\lib\async.js:558:23)

Below there's a fix attached.