rpc: Fix potentially segfaults

rpc: Fix potentially segfaults

We have to allocate first and if, and only if, it was successful we
can set the count. A segfault has occurred in
virNetServerServiceNewPostExecRestart() when VIR_ALLOC_N(svc->socks,
n) has failed, but svc->nsocsk = n was already set. Thus
virObejectUnref(svc) was called and therefore it was possible that
virNetServerServiceDispose was called => segmentation fault.  For
safeness NULL pointer check were added in
virNetServerServiceDispose().

Signed-off-by: Marc Hartmayer <mhartmay@linux.vnet.ibm.com>
Reviewed-by: Boris Fiuczynski <fiuczy@linux.vnet.ibm.com>
Reviewed-by: Bjoern Walk <bwalk@linux.vnet.ibm.com>