Generate error for no pid given to Genserver.reply (#4876)

Prior to this change it is possible to send any two-pair tuple to
`GenServer.reply/2` and it will happily return `:ok`, catching any
errors.  This leads to some hard to track-down bugs if you pass
it the wrong term.

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>

Author: benfalk

lib/elixir/lib/gen_server.ex

@@ -746,7 +746,7 @@ defmodule GenServer do
   @spec reply(from, term) :: :ok
   def reply(client, reply)
 
-  def reply({to, tag}, reply) do
+  def reply({to, tag}, reply) when is_pid(to) do
     try do
       send(to, {tag, reply})
       :ok