connection: Don't write past the end of the connection buffer

If a message was too big to fit in the connection buffer, the code
in wl_buffer_put would just write past the end of it.

I haven't seen any real world use case that would trigger this bug, but
it was possible to trigger it by sending a long enough string to the
wl_data_source.offer request.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=69267