bgpd: Prevent use after free

bgpd: Prevent use after free

When running bgp_always_compare_med, I am frequently seeing a crash
After running with valgrind I am seeing this and a invalid write
immediately after this as well.

==311743== Invalid read of size 2
==311743==    at 0x4992421: route_map_counter_decrement (routemap.c:3308)
==311743==    by 0x35664D: peer_route_map_unset (bgpd.c:7259)
==311743==    by 0x306546: peer_route_map_unset_vty (bgp_vty.c:8037)
==311743==    by 0x3066AC: no_neighbor_route_map (bgp_vty.c:8081)
==311743==    by 0x49078DE: cmd_execute_command_real (command.c:990)
==311743==    by 0x4907A63: cmd_execute_command (command.c:1050)
==311743==    by 0x490801F: cmd_execute (command.c:1217)
==311743==    by 0x49C5535: vty_command (vty.c:551)
==311743==    by 0x49C7459: vty_execute (vty.c:1314)
==311743==    by 0x49C97D1: vtysh_read (vty.c:2223)
==311743==    by 0x49BE5E2: event_call (event.c:1995)
==311743==    by 0x494786C: frr_run (libfrr.c:1204)
==311743==    by 0x1F7655: main (bgp_main.c:505)
==311743==  Address 0x9ec2180 is 64 bytes inside a block of size 120 free'd
==311743==    at 0x484B27F: free (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==311743==    by 0x495A1BA: qfree (memory.c:130)
==311743==    by 0x498D412: route_map_free_map (routemap.c:748)
==311743==    by 0x498D176: route_map_add (routemap.c:672)
==311743==    by 0x498D79B: route_map_get (routemap.c:857)
==311743==    by 0x499C256: lib_route_map_create (routemap_northbound.c:102)
==311743==    by 0x49702D8: nb_callback_create (northbound.c:1234)
==311743==    by 0x497107F: nb_callback_configuration (northbound.c:1578)
==311743==    by 0x4971693: nb_transaction_process (northbound.c:1709)
==311743==    by 0x496FCF4: nb_candidate_commit_apply (northbound.c:1103)
==311743==    by 0x496FE4E: nb_candidate_commit (northbound.c:1136)
==311743==    by 0x497798F: nb_cli_classic_commit (northbound_cli.c:49)
==311743==    by 0x4977B4F: nb_cli_pending_commit_check (northbound_cli.c:88)
==311743==    by 0x49078C1: cmd_execute_command_real (command.c:987)
==311743==    by 0x4907B44: cmd_execute_command (command.c:1068)
==311743==    by 0x490801F: cmd_execute (command.c:1217)
==311743==    by 0x49C5535: vty_command (vty.c:551)
==311743==    by 0x49C7459: vty_execute (vty.c:1314)
==311743==    by 0x49C97D1: vtysh_read (vty.c:2223)
==311743==    by 0x49BE5E2: event_call (event.c:1995)
==311743==    by 0x494786C: frr_run (libfrr.c:1204)
==311743==    by 0x1F7655: main (bgp_main.c:505)
==311743==  Block was alloc'd at
==311743==    at 0x484DA83: calloc (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==311743==    by 0x495A068: qcalloc (memory.c:105)
==311743==    by 0x498D0C8: route_map_new (routemap.c:646)
==311743==    by 0x498D128: route_map_add (routemap.c:658)
==311743==    by 0x498D79B: route_map_get (routemap.c:857)
==311743==    by 0x499C256: lib_route_map_create (routemap_northbound.c:102)
==311743==    by 0x49702D8: nb_callback_create (northbound.c:1234)
==311743==    by 0x497107F: nb_callback_configuration (northbound.c:1578)
==311743==    by 0x4971693: nb_transaction_process (northbound.c:1709)
==311743==    by 0x496FCF4: nb_candidate_commit_apply (northbound.c:1103)
==311743==    by 0x496FE4E: nb_candidate_commit (northbound.c:1136)
==311743==    by 0x497798F: nb_cli_classic_commit (northbound_cli.c:49)
==311743==    by 0x4977B4F: nb_cli_pending_commit_check (northbound_cli.c:88)
==311743==    by 0x49078C1: cmd_execute_command_real (command.c:987)
==311743==    by 0x4907B44: cmd_execute_command (command.c:1068)
==311743==    by 0x490801F: cmd_execute (command.c:1217)
==311743==    by 0x49C5535: vty_command (vty.c:551)
==311743==    by 0x49C7459: vty_execute (vty.c:1314)
==311743==    by 0x49C97D1: vtysh_read (vty.c:2223)
==311743==    by 0x49BE5E2: event_call (event.c:1995)
==311743==    by 0x494786C: frr_run (libfrr.c:1204)

Effectively the route_map that is being stored has been freed already
but we have not cleaned up properly yet.  Go through and clean the
code up by ensuring that the pointer actually exists instead of trusting
it does when doing the decrement operation.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>