Fix buffer overflow bug in deflate::core::BitBuffer::flush (#47)

* Fix buffer overflow bug in deflate::core::BitBuffer::flush

The problem is that this function does not check that there is enough space to fit a u64, it only checks that the starting position for the write is in bounds.
I have tried adding `assert!(pos.checked_add(8).unwrap() <= output.inner.get_ref().len());` before the unsafe part, but that regressed performance exactly as much as the fully safe version, so I went for the fully safe code.

* Support older rustc versions

miniz_oxide/src/deflate/core.rs

@@ -570,11 +570,9 @@ impl BitBuffer {
 
     fn flush(&mut self, output: &mut OutputBufferOxide) -> io::Result<()> {
         let pos = output.inner.position() as usize;
-        let inner = &mut ((*output.inner.get_mut())[pos]) as *mut u8 as *mut u64;
-        // # Unsafe
-        // TODO: check unsafety
-        unsafe {
-            ptr::write_unaligned(inner, self.bit_buffer.to_le());
+        { // isolation to please borrow checker
+            let inner = &mut (*output.inner.get_mut())[pos..pos+8];
+            inner.copy_from_slice(&u64_to_le_bytes(self.bit_buffer));
         }
         output.inner.seek(
             SeekFrom::Current((self.bits_in >> 3) as i64),
@@ -585,6 +583,13 @@ impl BitBuffer {
     }
 }
 
+#[inline]
+/// Copy of u64::to_le_bytes() that is only available starting at Rust 1.32,
+/// while we want to support older Rust versions
+fn u64_to_le_bytes(num: u64) -> [u8; 8] {
+    unsafe { mem::transmute(num.to_le()) }
+}
+
 /// A struct containing data about huffman codes and symbol frequencies.
 ///
 /// NOTE: Only the literal/lengths have enough symbols to actually use