Handle the case when SourceMapConsumer.originalPositionFor returns null source.

This happens when SourceMapConsumer does not have a valid position to map the input line and column. This is a change in mozilla/source-map starting from version 0.1.33

Fixes #436

package.json

@@ -16,7 +16,7 @@
     },
     "dependencies": {
         "async"      : "~0.2.6",
-        "source-map" : "~0.1.31",
+        "source-map" : "~0.1.33",
         "optimist"   : "~0.3.5",
         "uglify-to-browserify": "~1.0.0"
     },