ovsdb-client: fix memory leak in do_needs_conversion() and do_convert()

ovsdb-client: fix memory leak in do_needs_conversion() and do_convert()

Memory leak itself is not so important, however the problem is that
it is caused by forgetting to close rpc channel which might in
a long term lead to the leak of system resources.

Memory leak is reported by Valgrin running test suite and is expressed as:

==29472== 784 (600 direct, 184 indirect) bytes in 1 blocks are definitely lost in loss record 23 of 23
==29472==    at 0x4C2FB55: calloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==29472==    by 0x449D32: xcalloc (util.c:121)
==29472==    by 0x432147: jsonrpc_open (jsonrpc.c:87)
==29472==    by 0x40ABBE: open_jsonrpc (ovsdb-client.c:528)
==29472==    by 0x40ABBE: open_rpc (ovsdb-client.c:143)
==29472==    by 0x40AE50: do_needs_conversion (ovsdb-client.c:1670)
==29472==    by 0x405F76: main (ovsdb-client.c:282)

==29464== 784 (600 direct, 184 indirect) bytes in 1 blocks are definitely lost in loss record 23 of 23
==29464==    at 0x4C2FB55: calloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==29464==    by 0x449D32: xcalloc (util.c:121)
==29464==    by 0x432147: jsonrpc_open (jsonrpc.c:87)
==29464==    by 0x40ABBE: open_jsonrpc (ovsdb-client.c:528)
==29464==    by 0x40ABBE: open_rpc (ovsdb-client.c:143)
==29464==    by 0x40AF5A: do_convert (ovsdb-client.c:1644)
==29464==    by 0x405F76: main (ovsdb-client.c:282)

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