conf: Make really sure we don't access non-existing vCPUs again

conf: Make really sure we don't access non-existing vCPUs again

MinGW complained that we might be dereferencing a NULL pointer.  While
that can't be true, the logic certainly allows for that.

../../src/conf/domain_conf.c: In function 'virDomainDefPostParse':
../../src/conf/domain_conf.c:4224:18: error: potential null pointer dereference [-Werror=null-dereference]
         if (!vcpu->online && vcpu->cpumask) {
              ~~~~^~~~~~~~

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>