remove uneeded check

remix-lib/src/sourceMappingDecoder.js

@@ -216,7 +216,6 @@ function atIndex (index, mapping) {
     }
     current = current.split(':')
     if (current[2] === '-1') { // if the current step has -1 for the file attribute, we discard it
-      if (ret.file === undefined) ret = {} // if the file was not already set by a previous (meaning it is -1), we discard the whole step
       continue
     }
     if (ret.start === undefined && current[0] && current[0] !== '-1' && current[0].length) {