Another missing initialisation.

This one spotted in the old-fashioned way, by actually attempting a
Plink raw connection and wondering why it didn't seem to be reading
from standard input! Turns out 'bufsize' is uninitialised until the
first send, which can inhibit any stdin reading if it gets a large
enough nonsense value.

raw.c

@@ -146,6 +146,7 @@ static const char *raw_init(void *frontend_handle, void **backend_handle,
     raw->closed_on_socket_error = FALSE;
     *backend_handle = raw;
     raw->sent_console_eof = raw->sent_socket_eof = FALSE;
+    raw->bufsize = 0;
 
     raw->frontend = frontend_handle;