Closes GH-687 Don't read fs read stream if not open

lib/fs.js

@@ -921,6 +921,9 @@ ReadStream.prototype.resume = function() {
     this.buffer = null;
   }
 
+  // hasn't opened yet.
+  if (null == this.fd) return;
+
   this._read();
 };
 

test/simple/test-fs-read-stream.js

@@ -119,3 +119,8 @@ stream.on('data', function(chunk) {
 stream.on('end', function() {
   assert.equal('x', stream.data);
 });
+
+// pause and then resume immediately.
+var pauseRes = fs.createReadStream(rangeFile);
+pauseRes.pause();
+pauseRes.resume();