Fix compilation now EthernetClient equality operator is overloaded

Remove comparison of `m_client` against `NULL` as this causes the following compilation error since the addition of an overload for the equality operator in arduino/Arduino@ca37de4ba4ecbdb941f14ac1fe7dd40f3008af75:
```
Webduino/WebServer.h: In member function 'int WebServer::read()':
Webduino/WebServer.h:729: error: ambiguous overload for 'operator==' in '((WebServer*)this)->WebServer::m_client == 0'
Webduino/WebServer.h:729: note: candidates are: operator==(int, int) <built-in>
libraries\Ethernet\src/EthernetClient.h:27: note:                 virtual bool EthernetClient::operator==(const EthernetClient&)
```
This also initializes `m_client` with `MAX_SOCK_NUMBER` instead of `255` to make the boolean test of `m_client` work correctly.