schedule_output expects drain_queue_i to empty the queue (which means
transport->head_ == 0), but drain_queue_i doesn't change the head_ field.

The head_ isn't changed as the Message_Block it poits has length 0, and the
transported used is TAO_UIPMC_Transport.
This happens after TAO_UIPMC_Transport::send 

11      TAO_Block_Flushing_Strategy::schedule_output (TAO_Transport *transport)
12      {
13        while (!transport->queue_is_empty_i ())
14          {
15            int result = transport->drain_queue_i ();
16            if (result == -1)
17              return -1;
18          }
19        return 0;
20      }

It seems to happen when 2 multicast requests are sent at the same time, which
disturbs the marshalling generating a wrong packet

the stack trace is:
#0  TAO_Transport::drain_queue_i (this=0xb3602640) at Transport.cpp:831
#1  0xb7ae8594 in TAO_Block_Flushing_Strategy::schedule_output (this=0x8199fa0,
transport=0xb3602640) at Block_Flushing_Strategy.cpp:15
#2  0xb7af4c2e in TAO_Transport::send_synchronous_message_i (this=0xb3602640,
mb=0x0, max_wait_time=0x0) at Transport.cpp:444
#3  0xb7af6c59 in TAO_Transport::send_message_shared_i (this=0xb3602640,
stub=0xb3600500, message_semantics=0, message_block=0xb3602934,
    max_wait_time=0x0) at Transport.cpp:984
#4  0xb7af4a5c in TAO_Transport::send_message_shared (this=0xb3602640, stub=0x0,
message_semantics=0, message_block=0x0, max_wait_time=0x0)
    at Transport.cpp:406
#5  0xb7f518fc in TAO_UIPMC_Transport::send_message (this=0xb3602640,
stream=@0xb3600500, stub=0x0, message_semantics=0, max_wait_time=0x0)
    at CDR_Stream.inl:465
#6  0xb7f51884 in TAO_UIPMC_Transport::send_request (this=0xb3602640, stub=0x0,
orb_core=0x0, stream=@0x0, message_semantics=-1285543628,
    max_wait_time=0x0) at PortableGroup/UIPMC_Transport.cpp:554
#7  0xb7a6b2b2 in TAO::Remote_Invocation::send_message (this=0xb35ff6e0,
cdr=@0x0, message_semantics=576, max_wait_time=0x0)
    at Profile_Transport_Resolver.inl:38
#8  0xb7a6d013 in TAO::Synch_Oneway_Invocation::remote_oneway (this=0xb35ff6e0,
max_wait_time=0x1) at Synch_Invocation.cpp:707
#9  0xb7a6996d in TAO::Invocation_Adapter::invoke_oneway (this=0xb35ff920,
op=@0xb35ff82c, effective_target=@0xb35ff830, r=@0xb35ff7c0,
    max_wait_time=@0x0) at Invocation_Adapter.cpp:305
#10 0xb7a69705 in TAO::Invocation_Adapter::invoke_remote_i (this=0xb35ff920,
stub=0xb3600500, details=@0xb35ff880, effective_target=@0xb35ff830,
    max_wait_time=@0xb35ff82c) at Invocation_Adapter.cpp:218
#11 0xb7a691c5 in TAO::Invocation_Adapter::invoke_i (this=0xb35ff920,
stub=0xb3600500, details=@0xb35ff880) at Invocation_Adapter.cpp:83
#12 0xb7a69109 in TAO::Invocation_Adapter::invoke (this=0xb35ff920, ex_data=0x0,
ex_count=0) at Invocation_Adapter.cpp:44