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

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 'virDomainDefGetVcpuPinInfoHelper':
src/conf/domain_conf.c:1545:17: error: potential null pointer dereference [-Werror=null-dereference]
         if (vcpu->cpumask)
              ~~~~^~~~~~~~~
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

src/conf/domain_conf.c

@@ -1542,7 +1542,7 @@ virDomainDefGetVcpuPinInfoHelper(virDomainDefPtr def,
         virDomainVcpuDefPtr vcpu = virDomainDefGetVcpu(def, i);
         virBitmapPtr bitmap = NULL;
 
-        if (vcpu->cpumask)
+        if (vcpu && vcpu->cpumask)
             bitmap = vcpu->cpumask;
         else if (def->placement_mode == VIR_DOMAIN_CPU_PLACEMENT_MODE_AUTO &&
                  autoCpuset)