Fix compile now EthernetClient == is overloaded

Remove comparison of m_client against NULL as this causes a compilation
error since arduino/Arduino@ca37de4ba4ecbdb941f14ac1fe7dd40f3008af75.
Also initialize m_client with MAX_SOCK_NUMBER to make boolean test of
m_client work correctly.
Merge pull request #46 from ribbons/ethernetclient-compile-error

Fix compilation now EthernetClient equality operator is overloaded