VOM: handle null for iterator in dump cmd

Change-Id: I2e8743d70a8d8604d370218a73d5f37c2f7c4617
Signed-off-by: Neale Ranns <neale.ranns@cisco.com>
diff --git a/src/vpp-api/vom/dump_cmd.hpp b/src/vpp-api/vom/dump_cmd.hpp
index cc255ac..72fcdf5 100644
--- a/src/vpp-api/vom/dump_cmd.hpp
+++ b/src/vpp-api/vom/dump_cmd.hpp
@@ -62,9 +62,28 @@
    */
   virtual ~dump_cmd() {}
 
-  const_iterator begin() { return (m_dump->get_result_set().begin()); }
+  /**
+   * Constant iterator to the start of the records retunred during the dump
+   */
+  const_iterator begin()
+  {
+    /*
+     * m_dump is NULL during client UT when the commands are not issued.
+     */
+    if (!m_dump)
+      return const_iterator();
+    return (m_dump->get_result_set().begin());
+  }
 
-  const_iterator end() { return (m_dump->get_result_set().end()); }
+  /**
+   * Constant iterator to the end of the records retunred during the dump
+   */
+  const_iterator end()
+  {
+    if (!m_dump)
+      return const_iterator();
+    return (m_dump->get_result_set().end());
+  }
 
   /**
    * Wait for the issue of the command to complete