srbchannel: Check return value of pa_memblock_new_pool

In case PA_MEMPOOL_DISABLE is set, pa_memblock_new_pool can return
NULL. It does not make sense to set up a srbchannel without a shared
memory pool, so just fail in this case.

Reported-by: Alexander Patrakov <patrakov@gmail.com>
Signed-off-by: David Henningsson <david.henningsson@canonical.com>