Null pointer dereference in mrb_hash_dup

[clayton-shopify]

The following input demonstrates a crash:

class X < Hash
  def initialize
  end
end

X.new.dup

ASAN report:

==82906==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000004 (pc 0x0001078722a7 bp 0x7fff58422590 sp 0x7fff58422080 T0)
==82906==The signal is caused by a READ memory access.
==82906==Hint: address points to the zero page.
    #0 0x1078722a6 in mrb_hash_dup hash.c:241
    #1 0x107944c24 in mrb_vm_exec vm.c:1276
    #2 0x107939dbf in mrb_vm_run vm.c:829
    #3 0x10796c8d9 in mrb_top_run vm.c:2655
    #4 0x107a3dc65 in mrb_load_exec parse.y:5780
    #5 0x107a3e5b5 in mrb_load_file_cxt parse.y:5789
    #6 0x1077d60e6 in main mruby.c:227
    #7 0x7fffbbbba234 in start (libdyld.dylib:x86_64+0x5234)

==82906==Register values:
rax = 0x0000000000000004  rbx = 0x00007fff58422180  rcx = 0x000060400001e9d0  rdx = 0x0000000000000004
rdi = 0x0000100000000000  rsi = 0x0000100000000000  rbp = 0x00007fff58422590  rsp = 0x00007fff58422080
 r8 = 0x00001c0800003d00   r9 = 0x0000100000000000  r10 = 0x0000000000000080  r11 = 0x0000000000000000
r12 = 0x00007fff58422140  r13 = 0x00007fff58422160  r14 = 0x00007fff58422100  r15 = 0x00007fff58422120
AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV hash.c:241 in mrb_hash_dup
==82906==ABORTING
Abort trap: 6

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