assert on MySQL_Connection::assign_mshge when there is no server for the specified hostgroup

[renecannao]

This code needs major clean up and merged with changes for issue #79

(gdb) bt
#0  0x00007ffff6448925 in raise () from /lib64/libc.so.6
#1  0x00007ffff644a105 in abort () from /lib64/libc.so.6
#2  0x00007ffff6441a4e in __assert_fail_base () from /lib64/libc.so.6
#3  0x00007ffff6441b10 in __assert_fail () from /lib64/libc.so.6
#4  0x0000000000451548 in MySQL_Connection::assign_mshge (this=0x7ffff0828a00, hid=1) at mysql_connection.cpp:64
#5  0x00007ffff41bc760 in MySQL_Data_Stream::assign_mshge (this=0x7ffff087f800, hid=1) at mysql_data_stream.cpp:519
#6  0x00007ffff41be641 in MySQL_Session::handler (this=0x7ffff08bff00) at mysql_session.cpp:509
#7  0x00007ffff41b436e in Standard_MySQL_Thread::run (this=0x7ffff0817060) at Standard_MySQL_Thread.cpp:445
#8  0x00000000004301ef in mysql_worker_thread_func (arg=0x7ffff48177a0) at main.cpp:216
#9  0x00007ffff7bc79d1 in start_thread () from /lib64/libpthread.so.0
#10 0x00007ffff64feb6d in clone () from /lib64/libc.so.6