nsp: fix deref-after-null-check in nsp_ndr_push_prop_val_union

[COV]

Fixes: gromox-1.16-206-g1a461ff85

Author: jengelh

exch/nsp/nsp_ndr.cpp

@@ -1070,7 +1070,7 @@ static pack_result nsp_ndr_push_prop_val_union(NDR_PUSH *pndr,
 		TRY(nsp_ndr_push_binary(pndr, FLAG_CONTENT, &r->bin));
 		break;
 	case PT_CLSID:
-		if (r->pguid == nullptr)
+		if (r->pguid != nullptr)
 			TRY(nsp_ndr_push_flatuid(pndr, r->pguid));
 		break;
 	case PT_MV_SHORT: