ovsdb-server: fix memory leak while converting database

Memory leak happens while converting existing database into new
database according to the specified schema (ovsdb-client convert
new-schema). Memory leak was detected by valgrind while executing
functional test "schema conversion online - clustered"

==16202== 96 bytes in 6 blocks are definitely lost in loss record 326 of 399
==16202==    at 0x4C2DB8F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==16202==    by 0x44A5D4: xmalloc (util.c:138)
==16202==    by 0x4377A6: alloc_default_atoms (ovsdb-data.c:315)
==16202==    by 0x437F18: ovsdb_datum_init_default (ovsdb-data.c:918)
==16202==    by 0x413D82: ovsdb_row_create (row.c:59)
==16202==    by 0x40AA53: ovsdb_convert_table (file.c:220)
==16202==    by 0x40AA53: ovsdb_convert (file.c:275)
==16202==    by 0x416BE1: ovsdb_trigger_try (trigger.c:255)
==16202==    by 0x40D29E: ovsdb_jsonrpc_trigger_create (jsonrpc-server.c:1119)
==16202==    by 0x40D29E: ovsdb_jsonrpc_session_got_request (jsonrpc-server.c:986)
==16202==    by 0x40D29E: ovsdb_jsonrpc_session_run (jsonrpc-server.c:556)
==16202==    by 0x40D29E: ovsdb_jsonrpc_session_run_all (jsonrpc-server.c:586)
==16202==    by 0x40D29E: ovsdb_jsonrpc_server_run (jsonrpc-server.c:401)
==16202==    by 0x40682E: main_loop (ovsdb-server.c:209)
==16202==    by 0x40682E: main (ovsdb-server.c:460)

The problem was in ovsdb_datum_convert() function, which overrides
pointers to datum memory allocated in ovsdb_row_create() function.
Fix was done by freeing this memory before ovsdb_datum_convert()
is called.

Signed-off-by: Damijan Skvarc <damjan.skvarc@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

ovsdb/file.c

@@ -235,10 +235,14 @@ ovsdb_convert_table(struct ovsdb_txn *txn,
                 continue;
             }
 
+            ovsdb_datum_destroy(&dst_row->fields[dst_column->index],
+                                &dst_column->type);
+
             struct ovsdb_error *error = ovsdb_datum_convert(
                 &dst_row->fields[dst_column->index], &dst_column->type,
                 &src_row->fields[src_column->index], &src_column->type);
             if (error) {
+                ovsdb_datum_init_empty(&dst_row->fields[dst_column->index]);
                 ovsdb_row_destroy(dst_row);
                 return error;
             }