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
If the original source map doesn't specify a name for location then assume that the name hasn't changed from the original source. In that case, use the current name detected by UglifyJS' AST.
This is useful while compressing generated code; for example compressing JS
compiled by CoffeeScript (assuming you got a source map):
uglifyjs2 --in-source-map generated.js.map \
--source-map uglified.js.map \
-o uglified.js
The above assumes you have a "generated.js.map" file which is the source
mapping between your CoffeeScript and the generated.js (compiled output from
CoffeeScript). The name of the input file is not present in this example;
it will be fetched from the source map (but it can be passed manually too).
The output will be in "uglified.js" and the output map "uglified.js.map"
will actually map to the original CoffeeScript code, rather than to
generated.js.
