Fix Buffer instance check in Tap#setData

This should now work if Buffer is not defined

lib/utils.js

@@ -515,7 +515,7 @@ class Tap {
   }
 
   setData (buf, pos) {
-    if (buf instanceof Buffer) {
+    if (typeof Buffer === 'function' && buf instanceof Buffer) {
       buf = new Uint8Array(buf.buffer, buf.byteOffset, buf.length);
     }
     this.arr = buf;