Assume missing map names means name hasn't changed

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.

lib/sourcemap.js

@@ -67,8 +67,10 @@ function SourceMap(options) {
             source = info.source;
             orig_line = info.line;
             orig_col = info.column;
+            if (info.name) {
                 name = info.name;
             }
+        }
         generator.addMapping({
             generated : { line: gen_line + options.dest_line_diff, column: gen_col },
             original  : { line: orig_line + options.orig_line_diff, column: orig_col },