vlib: vfio code rework

Change-Id: I99cf3e7cc991aa7d32385a155c707a6516516117
Signed-off-by: Damjan Marion <damarion@cisco.com>
diff --git a/src/vlib/physmem_funcs.h b/src/vlib/physmem_funcs.h
index dbb8d9d..0013c2f 100644
--- a/src/vlib/physmem_funcs.h
+++ b/src/vlib/physmem_funcs.h
@@ -121,7 +121,8 @@
 vlib_physmem_free (vlib_main_t * vm, vlib_physmem_region_index_t idx,
 		   void *mem)
 {
-  return vm->os_physmem_free (vm, idx, mem);
+  if (mem)
+    vm->os_physmem_free (vm, idx, mem);
 }
 
 always_inline u64