Null pointer dereference in mrb_class_real

[clayton-shopify]

The following input demonstrates a crash:

BasicObject.prepend Enumerable

class BasicObject < Class
end

ASAN report:

ASAN:DEADLYSIGNAL
=================================================================
==26786==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x00010d1fdfbe bp 0x7ffee2a2e4f0 sp 0x7ffee2a2e4b0 T0)
==26786==The signal is caused by a READ memory access.
==26786==Hint: address points to the zero page.
    #0 0x10d1fdfbd in mrb_class_real class.c:1768
    #1 0x10d1fdc86 in mrb_vm_define_class class.c:314
    #2 0x10d2946c2 in mrb_vm_exec vm.c:2821
    #3 0x10d265c2b in mrb_vm_run vm.c:950
    #4 0x10d29d983 in mrb_top_run vm.c:3005
    #5 0x10d4596e7 in mrb_load_exec parse.y:5835
    #6 0x10d45a529 in mrb_load_file_cxt parse.y:5844
    #7 0x10d1c9b85 in main mruby.c:279
    #8 0x7fff76f6d014 in start (libdyld.dylib:x86_64+0x1014)

==26786==Register values:
rax = 0x0000100000000001  rbx = 0x00007ffee2a2e5c0  rcx = 0x0000000000000000  rdx = 0x0000100000000000
rdi = 0x000062f000002e00  rsi = 0x00007ffee2a2e500  rbp = 0x00007ffee2a2e4f0  rsp = 0x00007ffee2a2e4b0
 r8 = 0x00001fffdc545c00   r9 = 0x00007ffee2a2e004  r10 = 0x00007ffee2a2e0a0  r11 = 0xffffffffffffffe0
r12 = 0x00007ffee2a35bc0  r13 = 0x00007ffee2a35be0  r14 = 0x00007ffee2a2e580  r15 = 0x00007ffee2a2e5a0
AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV class.c:1768 in mrb_class_real
==26786==ABORTING
Abort trap: 6

This issue was reported by Daniel Teuchert, Cornelius Aschermann, Tommaso Frassetto and Tigist Abera (https://hackerone.com/pnoltof).