vlib: Counter free needs to NULL the allocated counter vector

otherwise the next time the counter is validated this is dangling.

Type: fix
Fixes: 58fd481d73

Signed-off-by: Neale Ranns <neale@graphiant.com>
Change-Id: Ifa8d5ff27175cf6dfb30cbf023fa3251fe5c780e
(cherry picked from commit 80c0ae2437)

src/plugins/unittest/counter_test.c

@@ -89,6 +89,9 @@ test_simple_counter_expand (vlib_main_t *vm)
       epoch = new_epoch;
     }
 
+  vlib_free_simple_counter (&counter);
+  vlib_validate_simple_counter (&counter, 0);
+
   return 0;
 }
 
@@ -136,6 +139,9 @@ test_combined_counter_expand (vlib_main_t *vm)
       epoch = new_epoch;
     }
 
+  vlib_free_combined_counter (&counter);
+  vlib_validate_combined_counter (&counter, 0);
+
   return 0;
 }