object: Get rid of "warning: cast increases required alignment of target type"

object: Get rid of "warning: cast increases required alignment of target type"

On ARM, pa_object has less strict alignment requirements
than e.g. pa_sink and pa_source, so when pa_object is cast
to pa_sink, the compiler thinks that it's unsafe. In this
case, however, the pointer given to pa_sink_ref() was a
pa_sink pointer to begin with, so casting it first to
pa_object and then back to pa_sink is entirely safe.

This particular source of warnings is extremely annoying,
because this message is printed for any compilation unit
that includes sink.h, source.h or any other header that
defines a class, and the message tends to get printed
multiple times for one compilation unit:

In file included from ./pulsecore/source-output.h:37:0,
                 from ./pulsecore/source.h:49,
                 from ./pulsecore/sink.h:40,
                 from ./pulsecore/core.h:50,
                 from daemon/daemon-conf.h:31,
                 from daemon/cmdline.h:25,
                 from daemon/cmdline.c:38:
./pulsecore/sink-input.h: In function 'pa_sink_input_ref':
./pulsecore/sink-input.h:245:1: warning: cast increases required alignment of target type [-Wcast-align]