vvmx: fix intended assignment

From what I can tell, this was supposed to be an assignment (not warned
about by the compiler due to -Wno-unused, which is about to be removed).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/hvm/vmx/vvmx.c
+++ b/xen/arch/x86/hvm/vmx/vvmx.c
@@ -581,7 +581,7 @@ static void nvmx_purge_vvmcs(struct vcpu
     __clear_current_vvmcs(v);
     if ( nvcpu->nv_vvmcxaddr != VMCX_EADDR )
         hvm_unmap_guest_frame(nvcpu->nv_vvmcx);
-    nvcpu->nv_vvmcx == NULL;
+    nvcpu->nv_vvmcx = NULL;
     nvcpu->nv_vvmcxaddr = VMCX_EADDR;
     for (i=0; i<2; i++) {
         if ( nvmx->iobitmap[i] ) {