Null pointer dereference in mrb_obj_iv_get

[clayton-shopify]

The following input demonstrates a crash:

h = {}
a =String [a]

It seems the problem was caused by abed375.

ASAN report:

ASAN:DEADLYSIGNAL
=================================================================
==20379==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000018 (pc 0x00010d2b3ce7 bp 0x7fff52a78210 sp 0x7fff52a780a0 T0)
==20379==The signal is caused by a READ memory access.
==20379==Hint: address points to the zero page.
    #0 0x10d2b3ce6 in mrb_obj_iv_get (mruby:x86_64+0x100140ce6)
    #1 0x10d1b1c6c in mrb_class_path (mruby:x86_64+0x10003ec6c)
    #2 0x10d1b34af in mrb_class_name (mruby:x86_64+0x1000404af)
    #3 0x10d1b3f92 in mrb_obj_classname (mruby:x86_64+0x100040f92)
    #4 0x10d24f9aa in mrb_any_to_s (mruby:x86_64+0x1000dc9aa)
    #5 0x10d2185cf in mrb_method_missing (mruby:x86_64+0x1000a55cf)
    #6 0x10d2e028b in mrb_vm_exec (mruby:x86_64+0x10016d28b)
    #7 0x10d2d5ae3 in mrb_vm_run (mruby:x86_64+0x100162ae3)
    #8 0x10d2cd97e in mrb_run (mruby:x86_64+0x10015a97e)
    #9 0x10d2cb304 in mrb_funcall_with_block (mruby:x86_64+0x100158304)
    #10 0x10d2c7ed7 in mrb_funcall_argv (mruby:x86_64+0x100154ed7)
    #11 0x10d24c9be in convert_type (mruby:x86_64+0x1000d99be)
    #12 0x10d24dada in mrb_check_convert_type (mruby:x86_64+0x1000daada)
    #13 0x10d381db7 in mrb_f_string (mruby:x86_64+0x10020edb7)
    #14 0x10d2e1296 in mrb_vm_exec (mruby:x86_64+0x10016e296)
    #15 0x10d2d5ae3 in mrb_vm_run (mruby:x86_64+0x100162ae3)
    #16 0x10d309d1f in mrb_top_run (mruby:x86_64+0x100196d1f)
    #17 0x10d3dbdd5 in mrb_load_exec (mruby:x86_64+0x100268dd5)
    #18 0x10d3dc725 in mrb_load_file_cxt (mruby:x86_64+0x100269725)
    #19 0x10d174f46 in main mruby.c:227
    #20 0x7fffe5638234 in start (libdyld.dylib:x86_64+0x5234)

==20379==Register values:
rax = 0x0000000000000018  rbx = 0x00007fff52a78100  rcx = 0x000061400000a440  rdx = 0x00001fffea54f014
rdi = 0x00007fff52a780c0  rsi = 0x0000000000000000  rbp = 0x00007fff52a78210  rsp = 0x00007fff52a780a0
 r8 = 0x00007fff52a780e0   r9 = 0x00000000000002a3  r10 = 0x0000000000000018  r11 = 0x0000100000000003
r12 = 0x00007fff52a78260  r13 = 0x00007fff52a78280  r14 = 0x0000100000000000  r15 = 0x00007fff52a78240
AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV (mruby:x86_64+0x100140ce6) in mrb_obj_iv_get
==20379==ABORTING
Abort trap: 6

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