fix parse error when using fixed-size packet buf

hmm, not sure when this bug was introduced... Well, it's fixed now, so
that's good.

src/gdbstub_impl/mod.rs

@@ -192,10 +192,7 @@ impl<T: Target, C: Connection> GdbStubImpl<T, C> {
             buf.push(conn.read().map_err(Error::ConnectionRead)?)?;
         }
 
-        drop(buf);
-
-        let len = pkt_buf.len();
-        match Packet::from_buf(target, &mut pkt_buf.as_mut()[..len]) {
+        match Packet::from_buf(target, pkt_buf.as_mut()) {
             Ok(packet) => Ok(packet),
             Err(e) => {
                 // TODO: preserve this context within Error::PacketParse

src/protocol/packet.rs

@@ -44,7 +44,7 @@ impl<'a> PacketBuf<'a> {
         // split buffer into body and checksum components
         let (body, checksum) = pkt_buf.split_at_mut(end_of_body);
         let body = &mut body[1..]; // skip the '$'
-        let checksum = &mut checksum[1..]; // skip the '#'
+        let checksum = &mut checksum[1..][..2]; // skip the '#'
 
         // validate the checksum
         let checksum = decode_hex(checksum).map_err(|_| PacketParseError::MalformedChecksum)?;