Null pointer dereference in OP_RESCUE

[clayton-shopify]

The following input demonstrates a crash:

begin
rescue => a
end

begin
  b
rescue begin
    c ""
  rescue => d
    0
  ensure
  end
end

mrb->exc ends up NULL here:

mruby/src/vm.c

Line 1096 in f8b31a0

exc = mrb_obj_value(mrb->exc);

This looks to be related to #3487

ASAN report:

==63757==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x00010572b22c bp 0x7fff5a6286f0 sp 0x7fff5a628580 T0)
    #0 0x10572b22b in mrb_obj_value (mruby+0x10015d22b)
    #1 0x10573aef0 in mrb_vm_exec (mruby+0x10016cef0)
    #2 0x1057342f9 in mrb_vm_run (mruby+0x1001662f9)
    #3 0x105766929 in mrb_top_run (mruby+0x100198929)
    #4 0x105835b65 in mrb_load_exec (mruby+0x100267b65)
    #5 0x105836975 in mrb_load_file_cxt (mruby+0x100268975)
    #6 0x1055d0936 in main mruby.c:227
    #7 0x7fffaf458254 in start (libdyld.dylib+0x5254)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV (mruby+0x10015d22b) in mrb_obj_value
==63757==ABORTING
Abort trap: 6

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