fix distributed monitor(Key, standby) when the name is not yet registered

[ten0s]

Steps to reproduce the bug in original version:

INIT:

node1:
erl -sname n1 -pa ebin -pa deps/*/ebin

node2:
erl -sname n2 -pa ebin -pa deps/*/ebin

node1:
net_adm:ping('n2@carcharodon'), application:ensure_started(gproc), gproc_dist:start_link(all).

node2:
net_adm:ping('n1@carcharodon'), application:ensure_started(gproc), gproc_dist:start_link(all).

TEST:

node1:
gproc:monitor({n, g, test}, standby).

=ERROR REPORT==== 7-May-2016::21:09:29 ===
** Generic leader gproc_dist terminating
** Last message in was {'$leader_call',
{<0.45.0>,#Ref<0.0.2.214>},
{monitor,{n,g,test},<0.45.0>,standby}}
** When Server state == {state,false,true,[]}
** Reason for termination ==
** {function_clause,[{gproc_dist,insert_reg,
[[],
{n,g,test},
undefined,<0.45.0>,
{failover,<0.45.0>}],
[{file,"src/gproc_dist.erl"},{line,1027}]},
{gproc_dist,handle_leader_call,4,
[{file,"src/gproc_dist.erl"},{line,397}]},
{gen_leader,handle_msg,4,
[{file,"src/gen_leader.erl"},{line,1103}]},
{proc_lib,init_p_do_apply,3,
[{file,"proc_lib.erl"},{line,240}]}]}
** exception exit: function_clause
in function gproc_dist:insert_reg/5
called as gproc_dist:insert_reg([],{n,g,test},undefined,<0.45.0>,{failover,<0.45.0>})
in call from gproc_dist:handle_leader_call/4 (src/gproc_dist.erl, line 397)
in call from gen_leader:handle_msg/4 (src/gen_leader.erl, line 1103)
in call from proc_lib:init_p_do_apply/3 (proc_lib.erl, line 240)

With fix applied:

INIT:
...
TEST:

node1:
gproc:monitor({n, g, test}, standby).

Ref<0.0.2.234>

flush().
Shell got {gproc,{failover,<0.45.0>},#Ref<0.0.2.234>,{n,g,test}}

node2:
gproc:where({n, g, test}).
<15262.45.0>

[uwiger]

Sorry about the very long delay. Merged. Thanks.