Gossip: Segfault in active set txbuild

[ravyu-jump]

Two independent reports of this occurring on testnet. Unable to repro and may be fixed by #6753 and/or #7418

GDB Backtrace provided by @jherrera-jump

#0  0x0000555555b88d0e in txbuild_flush (now=1763446927381455877, dest_addr=..., stem=0x1bdd2f7fcd40, txbuild=0x3abf8ac46b80, gossip=0x3abeb9c02700) at src/flamenco/gossip/fd_gossip.c:302
302	      gossip->metrics->crds_tx_push[ txbuild->crds[ i ].tag ]++;
[Current thread is 1 (LWP 3382407)]
Missing separate debuginfos, use: dnf debuginfo-install glibc-2.34-168.el9_6.24.x86_64
(gdb) bt
#0  0x0000555555b88d0e in txbuild_flush (now=1763446927381455877, dest_addr=..., stem=0x1bdd2f7fcd40, txbuild=0x3abf8ac46b80, gossip=0x3abeb9c02700) at src/flamenco/gossip/fd_gossip.c:302
#1  active_push_set_flush (gossip=gossip@entry=0x3abeb9c02700, pset=pset@entry=0x3abf8acc5498, idx=<optimized out>, stem=stem@entry=0x1bdd2f7fcd40, now=now@entry=1763446927381455877)
    at src/flamenco/gossip/fd_gossip.c:327
#2  0x0000555555b8b091 in flush_stale_push_states (now=1763446927381455877, stem=0x1bdd2f7fcd40, gossip=0x3abeb9c02700) at src/flamenco/gossip/fd_gossip.c:904
#3  fd_gossip_advance (gossip=0x3abeb9c02700, now=1763446927381455877, stem=stem@entry=0x1bdd2f7fcd40) at src/flamenco/gossip/fd_gossip.c:914
#4  0x0000555555a2e289 in after_credit (ctx=ctx@entry=0x3abeb9c01000, stem=stem@entry=0x1bdd2f7fcd40, opt_poll_in=opt_poll_in@entry=0x1bdd2f7fcd34, charge_busy=charge_busy@entry=0x1bdd2f7fcd30)
    at src/discof/gossip/fd_gossip_tile.c:179
#5  0x0000555555a2f2a4 in stem_run1 (burst=65537, lazy=1000, ctx=<optimized out>, scratch=0x1bdd2f7fc800, rng=<synthetic pointer>, _cons_fseq=0x1bdd2f7fe9b0, _cons_out=0x1bdd2f7fe1b0,
    cons_cnt=<optimized out>, out_mcache=0x1bdd2f7fd9b0, out_cnt=<optimized out>, in_fseq=0x1bdd2f7fcdb0, in_mcache=0x1bdd2f7fd1b0, in_cnt=<optimized out>)
    at src/discof/gossip/../../disco/stem/fd_stem.c:546
#6  stem_run (topo=<optimized out>, tile=<optimized out>) at src/discof/gossip/../../disco/stem/fd_stem.c:776
#7  0x0000555555b12a8f in fd_topo_run_tile (topo=0x555559fbeb60 <config+576>, tile=0x55555a037848 <config+495400>, sandbox=0, keep_controlling_terminal=keep_controlling_terminal@entry=0, dumpable=1,
    uid=1011, gid=1011, allow_fd=19, wait=0x0, debugger=0x0, tile_run=0x1bdd2f7fff50) at src/disco/topo/fd_topo_run.c:153
#8  0x00005555559f88e0 in tile_main (_args=0x7fffffffb6f0) at src/app/shared/commands/run/run1.c:59
#9  0x00007ffff790e3f4 in clone () from /lib64/libc.so.6
(gdb) p txbuild->crds[ i ].tag
$1 = 544222583
(gdb) p i
$2 = 0
(gdb) p txbuild->crds[ i ].off
$3 = 44
(gdb) p txbuild->crds[ i ].sz
$4 = 141