Unrecorded SIGSEGV during replay

While attempting to replay trace julia-3 in https://julialang-dumps.s3.amazonaws.com/linux64/3824/rr-run_3824-gitsha_da727d3563-2021-09-26_10_41_36.tar.zst, encountered a crash in rr at global_time 27:

[FATAL /workspace/srcdir/rr/src/ReplaySession.cc:491:cont_syscall_boundary()]
 (task 21565 (rec:12113) at time 27)
 -> Assertion `false' failed to hold. Replay got unrecorded signal {signo:SIGSEGV,errno:SUCCESS,code:SEGV_MAPERR,addr:0x7fe971f79048}
Tail of trace dump:
[snip] ...
{
  real_time:69877284.393712 global_time:27, event:`SYSCALL: brk' (state:ENTERING_SYSCALL) tid:12113, ticks:189
rax:0xffffffffffffffda rbx:0x400040 rcx:0xffffffffffffffff rdx:0x0 rsi:0x4007e0 rdi:0x0 rbp:0x7fe971d587c0 rsp:0x7ffe547dd468 r8:0x6fffd000 r9:0x37f r10:0x64 r11:0x246 r12:0xa r13:0x7ffe547dd7e9 r14:0x0 r15:0x1000 rip:0x7fe971d6cb8a eflags:0x246 cs:0x33 ss:0x2b ds:0x0 es:0x0 fs:0x0 gs:0x0 orig_rax:0xc fs_base:0x0 gs_base:0x0
}
{
  real_time:69877284.393746 global_time:28, event:`SYSCALL: brk' (state:EXITING_SYSCALL) tid:12113, ticks:189
rax:0x14df000 rbx:0x400040 rcx:0xffffffffffffffff rdx:0x0 rsi:0x4007e0 rdi:0x0 rbp:0x7fe971d587c0 rsp:0x7ffe547dd468 r8:0x6fffd000 r9:0x37f r10:0x64 r11:0x246 r12:0xa r13:0x7ffe547dd7e9 r14:0x0 r15:0x1000 rip:0x7fe971d6cb8a eflags:0x246 cs:0x33 ss:0x2b ds:0x0 es:0x0 fs:0x0 gs:0x0 orig_rax:0xc fs_base:0x0 gs_base:0x0
  { map_file:"<ZERO>", addr:0x14df000, length:(nil), prot_flags:"---p", file_offset:0x0, device:0, inode:0, data_file:"", data_offset:0x0, file_size:0x0 }
}

Replays fine with replay -a, so we believe there is an issue with the replayer rather than the trace itself

CC: @Keno

Start at ReplaySession.cc:491 in cont_syscall_boundary() and attempt to replay the linked julia-3 trace, comparing normal replay with replay -a. Use the global-time-27 brk events and the unrecorded SIGSEGV details as the initial reproduction point. Done means identifying and correcting the replayer behavior so this trace replays without the unrecorded signal.