memif: fix crash with zero-copy slave

Type: fix
Change-Id: I456cc0b0a6f2dc32b14791baf9d4a7f67279e8df
Signed-off-by: Damjan Marion <damarion@cisco.com>
diff --git a/src/plugins/memif/node.c b/src/plugins/memif/node.c
index 102d93d..c7ad974 100644
--- a/src/plugins/memif/node.c
+++ b/src/plugins/memif/node.c
@@ -809,7 +809,7 @@
       d[6].offset = (u64) vlib_get_buffer (vm, bi[6])->data - offset;
       d[7].offset = (u64) vlib_get_buffer (vm, bi[7])->data - offset;
 
-      slot += 8;
+      slot = (slot + 8) & mask;
       n_alloc -= 8;
       continue;