fs: handle fractional or NaN ReadStream buffer size

Fixes #2320.

lib/fs.js

@@ -1092,7 +1092,7 @@ ReadStream.prototype._read = function() {
   // thread pool another read() finishes up the pool, and allocates a new
   // one.
   var thisPool = pool;
-  var toRead = Math.min(pool.length - pool.used, this.bufferSize);
+  var toRead = Math.min(pool.length - pool.used, ~~this.bufferSize);
   var start = pool.used;
 
   if (this.pos !== undefined) {

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

@@ -133,6 +133,15 @@ file5.on('end', function() {
   assert.equal(file5.data, 'yz\n');
 });
 
+// https://github.com/joyent/node/issues/2320
+var file6 = fs.createReadStream(rangeFile, {bufferSize: 1.23, start: 1});
+file6.data = '';
+file6.on('data', function(data) {
+  file6.data += data.toString('utf-8');
+});
+file6.on('end', function() {
+  assert.equal(file6.data, 'yz\n');
+});
 
 assert.throws(function() {
   fs.createReadStream(rangeFile, {start: 10, end: 2});