vpnor: Fix SEGV fault in vpnor

Table object may throw error hence initialize table pointer to null so
that vpnor_free() doesn't free the junk pointer.
In vpnor_free() do not free table pointer if its null.

Added testcase to make sure the issue is fixed.
```
PASS: vpnor/test/toc_missing_file_reset
```

Change-Id: Ic6f4a7d163cf786d71b810190ab6ede0cd3a0717
Signed-off-by: Ninad Palsule <ninad@linux.ibm.com>
diff --git a/vpnor/backend.cpp b/vpnor/backend.cpp
index 3866c57..d081b13 100644
--- a/vpnor/backend.cpp
+++ b/vpnor/backend.cpp
@@ -85,6 +85,9 @@
     try
     {
         priv->vpnor = new vpnor_partition_table;
+        // Table object may throw error hence initialize table pointer
+        // to null so that no one try to free the junk pointer.
+        priv->vpnor->table = NULL;
         priv->vpnor->table =
             new openpower::virtual_pnor::partition::Table(backend);
     }