Null pointer dereference in kh_get_iv

[clayton-shopify]

The following input demonstrates a crash:

class << "0"
  class M e
  end
end

ASAN report:

==99582==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000030 (pc 0x00010b1b02ff bp 0x7fff54b85a70 sp 0x7fff54b85990 T0)
==99582==The signal is caused by a READ memory access.
==99582==Hint: address points to the zero page.
    #0 0x10b1b02fe in kh_get_iv (mruby:x86_64+0x1001422fe)
    #1 0x10b1b30fe in iv_get (mruby:x86_64+0x1001450fe)
    #2 0x10b1b2f2a in mrb_obj_iv_get (mruby:x86_64+0x100144f2a)
    #3 0x10b0af9ac in mrb_class_path (mruby:x86_64+0x1000419ac)
    #4 0x10b0afd2e in mrb_class_path (mruby:x86_64+0x100041d2e)
    #5 0x10b0c02e2 in mrb_mod_to_s (mruby:x86_64+0x1000522e2)
    #6 0x10b1c9d10 in mrb_funcall_with_block (mruby:x86_64+0x10015bd10)
    #7 0x10b1c6d47 in mrb_funcall_argv (mruby:x86_64+0x100158d47)
    #8 0x10b117982 in mrb_method_missing (mruby:x86_64+0x1000a9982)
    #9 0x10b1dd889 in mrb_vm_exec (mruby:x86_64+0x10016f889)
    #10 0x10b1d3ad9 in mrb_vm_run (mruby:x86_64+0x100165ad9)
    #11 0x10b206549 in mrb_top_run (mruby:x86_64+0x100198549)
    #12 0x10b2d7805 in mrb_load_exec (mruby:x86_64+0x100269805)
    #13 0x10b2d8615 in mrb_load_file_cxt (mruby:x86_64+0x10026a615)
    #14 0x10b0702e6 in main mruby.c:227
    #15 0x7fffbbbba234 in start (libdyld.dylib:x86_64+0x5234)

==99582==Register values:
rax = 0x0000000000000030  rbx = 0x00007fff54b85b60  rcx = 0x0000100000000000  rdx = 0x0000000000000887
rdi = 0x0000100000000006  rsi = 0x0000000000000030  rbp = 0x00007fff54b85a70  rsp = 0x00007fff54b85990
 r8 = 0x00007fff54b85b00   r9 = 0x00000000000002a3  r10 = 0x000062f000002128  r11 = 0x00001c5e00000425
r12 = 0x00007fff54b85cc0  r13 = 0x00007fff54b85ce0  r14 = 0x0000100000000000  r15 = 0x00007fff54b85ca0
AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV (mruby:x86_64+0x1001422fe) in kh_get_iv
==99582==ABORTING
Abort trap: 6

This issue was reported by Dinko Galetic and Denis Kasak (https://hackerone.com/dgaletic).