Source with inline sourcemaps

Hi,

I'm getting a different behavior with the same sourcemap either provided as external file or inline.
The external works well and provides an absolute path, the inline one just gives me the raw declaration.

Shouldn't [this line](https://github.com/evanw/node-source-map-support/blob/636b1b28e42c7cd3aa7fdfd9949ecd5bb9aef701/source-map-support.js#L148) be `sourceMap.url || position.source` ? At least it works consistently on my test.

Read source-map-support.js at line 148 and compare how external and inline sourcemaps populate sourceMap.url and position.source. Reproduce both cases from the report, then verify that the selected source path is consistent and absolute where expected.

Written by the indexing model from the issue text.