fix crash in url util function due to undefined root value: sourceRoot is checked everywhere using != null except in 1(one) spot, which caused a crash in my test set, while debugging indexed map issues with the origin API. (mozilla#394)

Author: GerHobbelt

lib/source-map-generator.js

@@ -71,7 +71,7 @@ class SourceMapGenerator {
     });
     aSourceMapConsumer.sources.forEach(function(sourceFile) {
       let sourceRelative = sourceFile;
-      if (sourceRoot !== null) {
+      if (sourceRoot != null) {
         sourceRelative = util.relative(sourceRoot, sourceFile);
       }