fix: empty shallowSlice return

Buffer.slice() and BL.slice() both support returning an empty dataset when start and end are both the same. We were defaulting the end arg assuming we’d always have a truthy value for end, when 0 is valid.

bl.js

@@ -203,13 +203,16 @@ BufferList.prototype.copy = function copy (dst, dstStart, srcStart, srcEnd) {
 
 BufferList.prototype.shallowSlice = function shallowSlice (start, end) {
   start = start || 0
-  end = end || this.length
+  end = typeof end !== 'number' ? this.length : end
 
   if (start < 0)
     start += this.length
   if (end < 0)
     end += this.length
 
+  if (start === end) {
+    return new BufferList()
+  }
   var startOffset = this._offset(start)
     , endOffset = this._offset(end)
     , buffers = this._bufs.slice(startOffset[0], endOffset[0] + 1)

test/test.js

@@ -649,6 +649,20 @@ tape('shallow slice does not make a copy', function (t) {
   t.equal(bl.toString(), 'hhhhhhhh')
 })
 
+tape('shallow slice with 0 length', function (t) {
+  t.plan(1)
+  var buffers = [Buffer.from('First'), Buffer.from('Second'), Buffer.from('Third')]
+  var bl = (new BufferList(buffers)).shallowSlice(0, 0)
+  t.equal(bl.length, 0)
+})
+
+tape('shallow slice with 0 length from middle', function (t) {
+  t.plan(1)
+  var buffers = [Buffer.from('First'), Buffer.from('Second'), Buffer.from('Third')]
+  var bl = (new BufferList(buffers)).shallowSlice(10, 10)
+  t.equal(bl.length, 0)
+})
+
 tape('duplicate', function (t) {
   t.plan(2)