codec: Fix panic in LengthDelimitedCodec::encode (#682)

Fixes: #681 

## Motivation

Currently, a potential panic exists in `LengthDelimitedCodec::encode`.
Writing the length field to the `dst` buffer can exceed the buffer
capacity, as `BufMut::put_uint_{le,be}` doesn't reserve more capacity. 

## Solution

This branch adds a call to `dst.reserve` to ensure that there's 
sufficient remaining buffer capacity to hold the length field and
the frame, prior to writing the length field. Previously, capacity
was only reserved later in the function, when writing the frame
to the buffer, and we never reserved capacity for the length field.

I've also added a test that reproduces the issue. The test panics on
master, but passes after making this change.

Signed-off-by: Eliza Weisman <[email protected]>

src/codec/length_delimited.rs

@@ -578,6 +578,10 @@ impl Encoder for LengthDelimitedCodec {
             "provided length would overflow after adjustment",
         ))?;
 
+        // Reserve capacity in the destination buffer to fit the frame and
+        // length field (plus adjustment).
+        dst.reserve(self.builder.length_field_len + n);
+
         if self.builder.length_field_is_big_endian {
             dst.put_uint_be(n as u64, self.builder.length_field_len);
         } else {