Closes GH-535 Immediate pause/resume race condition

Calling resume() immediately after calling pause() would trigger
a race condition where it would try to read() from a file
descriptor that was already being read from, causing an EBADF

lib/fs.js

@@ -838,7 +838,9 @@ ReadStream.prototype.setEncoding = function(encoding) {
 
 ReadStream.prototype._read = function() {
   var self = this;
-  if (!self.readable || self.paused) return;
+  if (!self.readable || self.paused || self.reading) return;
+
+  self.reading = true;
 
   if (!pool || pool.length - pool.used < kMinPoolSpace) {
     // discard the old pool. Can't add to the free list because
@@ -864,6 +866,7 @@ ReadStream.prototype._read = function() {
   }
 
   function afterRead(err, bytesRead) {
+    self.reading = false;
     if (err) {
       self.emit('error', err);
       self.readable = false;

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

@@ -42,8 +42,13 @@ file.addListener('open', function(fd) {
   callbacks.open++;
   assert.equal('number', typeof fd);
   assert.ok(file.readable);
-});
 
+  // GH-535
+  file.pause();
+  file.resume();
+  file.pause();
+  file.resume();
+});
 
 file.addListener('data', function(data) {
   assert.ok(data instanceof Buffer);