refs[t:2654] fix a NULL pointer deref found by valgrind brtloader-test-open

git-svn-id: file:///svn/toku/tokudb@20672 c7de825b-a66e-492c-adef-691d508d4ae1

newbrt/brtloader.c

@@ -315,10 +315,13 @@ void toku_brtloader_internal_destroy (BRTLOADER bl, BOOL is_error) {
         destroy_merge_fileset(&bl->fs[i]);
     toku_free(bl->fs);
 
+    if (bl->last_key) {
         for (int i=0; i < bl->N; i++) {
             toku_free(bl->last_key[i].data);
         }
         toku_free(bl->last_key);
+        bl->last_key = NULL;
+    }
 
     destroy_rowset(&bl->primary_rowset);