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
Merge pull request #439 from Arnavion/null-source-in-sourcemap

Handle the case when SourceMapConsumer.originalPositionFor returns null source.