[7.0]bgpd: Fix memory leak in json output of show commands

[donaldsharp]

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

[NetDEF-CI]

Continuous Integration Result: SUCCESSFUL

Congratulations, this patch passed basic tests

Tested-by: NetDEF / OpenSourceRouting.org CI System

CI System Testrun URL: https://ci1.netdef.org/browse/FRR-FRRPULLREQ-9896/

This is a comment from an automated CI system.
For questions and feedback in regards to this CI system, please feel free to email
Martin Winter - mwinter (at) opensourcerouting.org.

---

CLANG Static Analyzer Summary

• Github Pull Request 5449, comparing to Git base SHA 1178d1f

No Changes in Static Analysis warnings compared to base

67 Static Analyzer issues remaining.

See details at
https://ci1.netdef.org/browse/FRR-FRRPULLREQ-9896/artifact/shared/static_analysis/index.html

[LabN-CI]

💚 Basic BGPD CI results: SUCCESS, 0 tests failed

Results table

_	_
Result	SUCCESS git merge/5449 d6052e2
Date	12/01/2019
Start	20:35:09
Finish	21:00:44
Run-Time	25:35
Total	1815
Pass	1815
Fail	0
Valgrind-Errors	0
Valgrind-Loss	0
Details	vncregress-2019-12-01-20:35:09.txt
Log	autoscript-2019-12-01-20:35:54.log.bz2
Memory	434 435 368

For details, please contact louberger