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: NDaniel P. Berrange <berrange@redhat.com>

src/conf/domain_conf.c

@@ -4221,7 +4221,7 @@ virDomainDefRemoveOfflineVcpuPin(virDomainDefPtr def)
     for (i = 0; i < virDomainDefGetVcpusMax(def); i++) {
         vcpu = virDomainDefGetVcpu(def, i);
 
-        if (!vcpu->online && vcpu->cpumask) {
+        if (vcpu && !vcpu->online && vcpu->cpumask) {
             virBitmapFree(vcpu->cpumask);
             vcpu->cpumask = NULL;