Null pointer dereference in mrb_ary_ref

[clayton-shopify]

The following input to mirb demonstrates a crash:

w
Fiber.new do w end.resume
y=f=GC.start
g=s=t=().()
a=[D]
a

Note that this input must be given to mirb, not mruby.

ASAN report:

==24404==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x7fffc774bf3d bp 0x7fff57771860 sp 0x7fff57771860 T0)
    #0 0x7fffc774bf3c in _platform_memmove$VARIANT$Haswell (libsystem_platform.dylib+0x5f3c)
    #1 0x108821bf8 in __asan_memcpy (libclang_rt.asan_osx_dynamic.dylib+0x41bf8)
    #2 0x10848cee4 in mrb_ary_ref array.c:552
    #3 0x108496a28 in mrb_ary_aget array.c:753
    #4 0x1085f1d50 in mrb_vm_exec vm.c:1259
    #5 0x1085e73d9 in mrb_vm_run vm.c:823
    #6 0x1085e011e in mrb_run vm.c:2603
    #7 0x1085dda67 in mrb_funcall_with_block vm.c:451
    #8 0x1085dac57 in mrb_funcall_with_block vm.c:354
    #9 0x1085da437 in mrb_funcall_argv vm.c:461
    #10 0x1085d9ebe in mrb_funcall vm.c:339
    #11 0x108485c9b in p mirb.c:92
    #12 0x108484634 in main mirb.c:564
    #13 0x7fffc753b254 in start (libdyld.dylib+0x5254)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV (libsystem_platform.dylib+0x5f3c) in _platform_memmove$VARIANT$Haswell
==24404==ABORTING
Abort trap: 6

This issue was reported by https://hackerone.com/ston3