bgpd: Fix memory leak in json output of show commands

bgpd: Fix memory leak in json output of show commands

When dumping a large bit of table data via bgp_show_table
and if there is no information to display for a particular
`struct bgp_node *` the data allocated via json_object_new_array()
is leaked.  Not a big deal on small tables but if you have a full
bgp feed and issue a show command that does not match any of
the route nodes ( say `vtysh -c "show bgp ipv4 large-community-list FOO"`)
then we will leak memory.

Before code change and issuing the above show bgp large-community-list command 15-20 times:
Memory statistics for bgpd:
System allocator statistics:
  Total heap allocated:  > 2GB
  Holding block headers: 0 bytes
  Used small blocks:     0 bytes
  Used ordinary blocks:  > 2GB
  Free small blocks:     31 MiB
  Free ordinary blocks:  616 KiB
  Ordinary blocks:       0
  Small blocks:          0
  Holding blocks:        0

After:

Memory statistics for bgpd:
System allocator statistics:
  Total heap allocated:  924 MiB
  Holding block headers: 0 bytes
  Used small blocks:     0 bytes
  Used ordinary blocks:  558 MiB
  Free small blocks:     26 MiB
  Free ordinary blocks:  340 MiB
  Ordinary blocks:       0
  Small blocks:          0
  Holding blocks:        0

Please note the 340mb of free ordinary blocks is from the fact I issued a
`show bgp ipv4 uni json` command and generated a large amount of data.

Fixes: #5445
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>