Unsplit improvements (#173)

* Handle empty self and other for unsplit.
* Change extend() to extend_from_slice().

src/bytes.rs

@@ -295,6 +295,8 @@ pub struct BytesMut {
 #[cfg(target_endian = "little")]
 #[repr(C)]
 struct Inner {
+    // WARNING: Do not access the fields directly unless you know what you are
+    // doing. Instead, use the fns. See implementation comment above.
     arc: AtomicPtr<Shared>,
     ptr: *mut u8,
     len: usize,
@@ -304,6 +306,8 @@ struct Inner {
 #[cfg(target_endian = "big")]
 #[repr(C)]
 struct Inner {
+    // WARNING: Do not access the fields directly unless you know what you are
+    // doing. Instead, use the fns. See implementation comment above.
     ptr: *mut u8,
     len: usize,
     cap: usize,
@@ -1401,6 +1405,15 @@ impl BytesMut {
     pub fn unsplit(&mut self, other: BytesMut) {
         let ptr;
 
+        if other.is_empty() {
+            return;
+        }
+
+        if self.is_empty() {
+            *self = other;
+            return;
+        }
+
         unsafe {
             ptr = self.inner.ptr.offset(self.inner.len as isize); 
         }
@@ -1415,7 +1428,7 @@ impl BytesMut {
             self.inner.cap += other.inner.cap;
         }
         else {
-            self.extend(other);
+            self.extend_from_slice(&other);
         }
     }
 }