Fix two compiler warnings generated by updated toolchain

Fix two compiler warnings generated when building with warnings enabled (after the recent toolchain update):
```
libraries\Ethernet\src\utility\socket.cpp: In function 'uint16_t igmpsend(SOCKET, const uint8_t*, uint16_t)':
libraries\Ethernet\src\utility\socket.cpp:322:11: warning: variable 'status' set but not used [-Wunused-but-set-variable]
   uint8_t status=0;
           ^
hardware\arduino\avr\cores\arduino\Stream.cpp: In member function 'bool Stream::find(char*)':
hardware\arduino\avr\cores\arduino\Stream.cpp:78:30: warning: deprecated conversion from string constant to 'char*' [-Wwrite-strings]
   return findUntil(target, "");
                              ^
```
This change may remove eventual side-effects of reading SnSR register, so to be 100% sure it should be changed from:

```
status = W5100.readSnSR(s);
```

to

```
W5100.readSnSR(s);
```

BTW looking at the datasheet it seems that the status register is read-only and reading (or not reading) it should be transparent for any subsequent operation.