The following testcase crashes on mozilla-central revision d97cc5b9eeae (build with --enable-posix-nspr-emulation --enable-valgrind --enable-gczeal --disable-tests --disable-profiling --enable-debug --enable-optimize, run with --fuzzing-safe --ion-offthread-compile=off):

for (var i = 1; i < 100; ++i) {
    try {
        oomAtAllocation(i, i);
    } catch (e) {}
}
evalInWorker("");

Backtrace:

received signal SIGSEGV, Segmentation fault.
#0  mozilla::UniquePtr<js::gc::SweepAction<js::gc::GCRuntime*, js::FreeOp*, js::SliceBudget&>, JS::DeletePolicy<js::gc::SweepAction<js::gc::GCRuntime*, js::FreeOp*, js::SliceBudget&> > >::operator-> (this=<optimized out>) at dist/include/mozilla/UniquePtr.h:302
#1  sweepaction::SweepActionSequence<js::gc::GCRuntime*, js::FreeOp*, js::SliceBudget&>::init (count=<optimized out>, acts=<optimized out>, this=<optimized out>) at js/src/gc/GC.cpp:6259
#2  sweepaction::Sequence<js::gc::GCRuntime*, js::FreeOp*, js::SliceBudget&, mozilla::UniquePtr<...> (first=...) at js/src/gc/GC.cpp:6401
#3  js::gc::GCRuntime::initSweepActions (this=this@entry=0x7ffff4d936b8) at js/src/gc/GC.cpp:6453
#4  0x0000555555ff54c8 in js::gc::GCRuntime::init (this=this@entry=0x7ffff4d936b8, maxbytes=maxbytes@entry=8388608, maxNurseryBytes=maxNurseryBytes@entry=2097152) at js/src/gc/GC.cpp:1292
#5  0x0000555555beddeb in JSRuntime::init (this=this@entry=0x7ffff4d93000, cx=cx@entry=0x7ffff4d7a000, maxbytes=maxbytes@entry=8388608, maxNurseryBytes=maxNurseryBytes@entry=2097152) at js/src/vm/Runtime.cpp:205
#6  0x0000555555b210de in js::NewContext (maxBytes=8388608, maxNurseryBytes=2097152, parentRuntime=<optimized out>) at js/src/vm/JSContext.cpp:159
#7  0x0000555555860045 in WorkerMain (input=<optimized out>) at js/src/shell/js.cpp:3977
#8  0x0000555555863102 in js::detail::ThreadTrampoline<void (&)(WorkerInput*), WorkerInput*&>::callMain<0ul> (this=0x7ffff4d770f0) at js/src/threading/Thread.h:239
#9  js::detail::ThreadTrampoline<void (&)(WorkerInput*), WorkerInput*&>::Start (aPack=0x7ffff4d770f0) at js/src/threading/Thread.h:232
#10 0x00007ffff7bc16ba in start_thread (arg=0x7ffff68ff700) at pthread_create.c:333
#11 0x00007ffff6c2c41d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:109
rax	0x555557c32280	93825032987264
rbx	0x7ffff68fe530	140737330013488
rcx	0x555556b03410	93825014969360
rdx	0x0	0
rsi	0x7ffff6eeb770	140737336227696
rdi	0x7ffff6eea540	140737336223040
rbp	0x7ffff68fe5e0	140737330013664
rsp	0x7ffff68fe3f0	140737330013168
r8	0x7ffff6eeb770	140737336227696
r9	0x7ffff68ff700	140737330018048
r10	0x58	88
r11	0x7ffff6b927a0	140737332717472
r12	0x0	0
r13	0x6	6
r14	0x555557c32878	93825032988792
r15	0x7ffff4def600	140737301640704
rip	0x555555fe9e99 <js::gc::GCRuntime::initSweepActions()+2937>
=> 0x555555fe9e99 <js::gc::GCRuntime::initSweepActions()+2937>:	movl   $0x0,0x0
   0x555555fe9ea4 <js::gc::GCRuntime::initSweepActions()+2948>:	ud2

When initialising SweepActionSequence we need to check for allocation failure for any of the passed in SweepActions.

https://hg.mozilla.org/mozilla-central/rev/70b353cfbbf3

We're a week from 66 going to RC, so I think this can ride the trains. Feel free to nominate for Beta uplift if you feel strongly otherwise.