bin/lessc: Make sure path.dirname gets passed strings

Since nodejs/node@08085c49b6e, which will be part of Node.js v6.0,
functions from the `path` core module (like `dirname`) will require
the input to be a string.

Because `.sourceMapFullFilename` and `.sourceMapInputFilename` in
bin/lessc may be `undefined`, default to passing `'.'` to
`path.dirname` instead.

bin/lessc

@@ -316,8 +316,8 @@ function printUsage() {
     }
 
     if (sourceMapOptions.sourceMapRootpath === undefined) {
-        var pathToMap = path.dirname(sourceMapFileInline ? output : sourceMapOptions.sourceMapFullFilename),
-            pathToInput = path.dirname(sourceMapOptions.sourceMapInputFilename);
+        var pathToMap = path.dirname(sourceMapFileInline ? output : sourceMapOptions.sourceMapFullFilename || '.'),
+            pathToInput = path.dirname(sourceMapOptions.sourceMapInputFilename || '.');
         sourceMapOptions.sourceMapRootpath = path.relative(pathToMap, pathToInput);
     }