Set the add prepared statement to NULL after finalizing it.

tsk/hashdb/sqlite_index.cpp

@@ -720,6 +720,8 @@ sqlite_v1_close(TSK_IDX_INFO * idx_info)
         finalize_stmt(m_stmt);
     }
 
+    m_stmt = NULL;
+
     if (idx_info->idx_struct.idx_sqlite_v1->hIdx_sqlite) {
         sqlite3_close(idx_info->idx_struct.idx_sqlite_v1->hIdx_sqlite);
     }