[upstream] rtp-recv: fix crash on empty UDP packets (CVE-2014-3970)

[upstream] rtp-recv: fix crash on empty UDP packets (CVE-2014-3970)

Patch from:
https://cgit.freedesktop.org/pulseaudio/pulseaudio/commit/?id=26b9d22dd24c17eb118d0205bf7b02b75d435e3c

Original commit msg:
"On FIONREAD returning 0 bytes, we cannot return success, as the caller
(rtpoll_work_cb in module-rtp-recv.c) would then try to
pa_memblock_unref(chunk.memblock) and, because memblock is NULL, trigger
an assertion.

Also we have to read out the possible empty packet from the socket, so
that the kernel doesn't tell us again and again about it.

Change-Id: Ie7d2db03c793640d3e91b29442c186b52e024dff
Signed-off-by: Alexander E. Patrakov <patrakov@gmail.com>"