Null pointer dereference in mrb_obj_iv_get

[clayton-shopify]

The following input demonstrates a crash, when supplied to mirb: 239564.txt
ASAN report:

ASAN:DEADLYSIGNAL
=================================================================
==3010==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000018 (pc 0x000101c14f27 bp 0x7fff5e1151f0 sp 0x7fff5e115080 T0)
==3010==The signal is caused by a READ memory access.
==3010==Hint: address points to the zero page.
    #0 0x101c14f26 in mrb_obj_iv_get (mirb:x86_64+0x100142f26)
    #1 0x101b1376c in mrb_class_path (mirb:x86_64+0x10004176c)
    #2 0x101b14faf in mrb_class_name (mirb:x86_64+0x100042faf)
    #3 0x101b15a92 in mrb_obj_classname (mirb:x86_64+0x100043a92)
    #4 0x101bb0bfa in mrb_any_to_s (mirb:x86_64+0x1000debfa)
    #5 0x101b7982f in mrb_method_missing (mirb:x86_64+0x1000a782f)
    #6 0x101c411d8 in mrb_vm_exec (mirb:x86_64+0x10016f1d8)
    #7 0x101c36d64 in mrb_vm_run (mirb:x86_64+0x100164d64)
    #8 0x101c2ebfe in mrb_run (mirb:x86_64+0x10015cbfe)
    #9 0x101c2c544 in mrb_funcall_with_block (mirb:x86_64+0x10015a544)
    #10 0x101c29117 in mrb_funcall_argv (mirb:x86_64+0x100157117)
    #11 0x101badc0e in convert_type (mirb:x86_64+0x1000dbc0e)
    #12 0x101bae40b in mrb_convert_type (mirb:x86_64+0x1000dc40b)
    #13 0x101bd3b31 in mrb_str_to_str (mirb:x86_64+0x100101b31)
    #14 0x101c6e41b in mrb_str_format (mirb:x86_64+0x10019c41b)
    #15 0x101c6d879 in mrb_f_sprintf (mirb:x86_64+0x10019b879)
    #16 0x101c4211b in mrb_vm_exec (mirb:x86_64+0x10017011b)
    #17 0x101c36d64 in mrb_vm_run (mirb:x86_64+0x100164d64)
    #18 0x101ad4aa9 in main mirb.c:549
    #19 0x7fffe5638234 in start (libdyld.dylib:x86_64+0x5234)

==3010==Register values:
rax = 0x0000000000000018  rbx = 0x00007fff5e1150e0  rcx = 0x000061400000a440  rdx = 0x00001fffebc22a10
rdi = 0x00007fff5e1150a0  rsi = 0x0000000000000000  rbp = 0x00007fff5e1151f0  rsp = 0x00007fff5e115080
 r8 = 0x00007fff5e1150c0   r9 = 0x00000000000002aa  r10 = 0x0000000000000018  r11 = 0x0000100000000003
r12 = 0x00007fff5e115240  r13 = 0x00007fff5e115260  r14 = 0x0000100000000000  r15 = 0x00007fff5e115220
AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV (mirb:x86_64+0x100142f26) in mrb_obj_iv_get
==3010==ABORTING
Abort trap: 6

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