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

lib/sourcemap.js

@@ -64,6 +64,9 @@ function SourceMap(options) {
                 line: orig_line,
                 column: orig_col
             });
+            if (info.source === null) {
+                return;
+            }
             source = info.source;
             orig_line = info.line;
             orig_col = info.column;