Fix for SNC-3865

[rathnasabapathyv]

RCA for SNC-3865:

GET at "/restconf/data/openconfig-network-instance:network-instances/network-instance=default/protocols/protocol=BGP,bgp" is not worked, because while processing the GET request, REST-server got crashed as shown in this attachment ( RCA_RestServer_crash.txt )

Fix for SNC-3865:

YGOT memory for BGP container is not initialized. It was nill, since get is initiated at one container above. With this fix, allocating memory for BGP container. Also initialized the memory for GET callback to fill all data. For e.g. BGP RIB was not filled earlier.

Other fixes in this pull-request:

• Get BGP/global does not return BGP/global/state container, when IPV4 unicast is not enabled in any of the BGP-neighbors. It mandates atleast in one nbr IPV4 unicast should be enabled. That should not be the case. As shown in this attachment (global-state-get-err.txt)
• Also "show ip bgp summary" doesn't display the global BGP information like "Router-id" & "local-AS"
• Get at BGP container level does not return RIB information as well

UT-logs:

• Reverify the issue in JIRA (SNC-3865) : JIRA_issue_get.txt
• Single neighbor get : Single_nbr_get.txt
• BGP container get, that displays RIB info as well : JIRA_issue_get.txt
• BGP global container get to display global state info as well : BGP_ctnr_get.txt
• show ip bgp summary o/p with IPV4 unicast disabled in all nbrs: show_ip_bgp_summary_without_ipv4_in_all_nbrs.txt