Escape null characters as \x00

Since \0 might be mistakenly interpreted as octal if followed by a
number and using literal null is in some cases interpreted as end of
string, escape null as \x00.

lib/output.js

@@ -96,7 +96,7 @@ function OutputStream(options) {
               case "\u2029": return "\\u2029";
               case '"': ++dq; return '"';
               case "'": ++sq; return "'";
-              case "\0": return "\\0";
+              case "\0": return "\\x00";
             }
             return s;
         });