connection: return error on buffer-overflow during read

wl_connection_read() assumes that the caller dispatched all messages
before calling it. wl_buffer_put_iov() does only provide enough room so we
fill the buffer. So the only case when the buffer overflows, is when a
previous read filled up the buffer but we couldn't parse a single message
from it. In this case, the client sent a message bigger than our buffer
and we should return an error and close the connection.

krh: Edited from Davids original patch to just check that the buffer
 isn't full before we try reading into it.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>