Revert "hw/virtio/virtio-pmem: Replace impossible check by assertion"

This reverts commit 5960f254dbb46f0c7a9f5f44bf4d27c19c34cb97 since the
previous commit made this situation possible again.

Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
diff --git a/hw/virtio/virtio-pmem.c b/hw/virtio/virtio-pmem.c
index cc24812..c3512c2 100644
--- a/hw/virtio/virtio-pmem.c
+++ b/hw/virtio/virtio-pmem.c
@@ -147,7 +147,10 @@
 static MemoryRegion *virtio_pmem_get_memory_region(VirtIOPMEM *pmem,
                                                    Error **errp)
 {
-    assert(pmem->memdev);
+    if (!pmem->memdev) {
+        error_setg(errp, "'%s' property must be set", VIRTIO_PMEM_MEMDEV_PROP);
+        return NULL;
+    }
 
     return &pmem->memdev->mr;
 }