Null pointer dereference in OP_CALL

[clayton-shopify]

The following input demonstrates a crash:

class X < Proc
1.times{
    super()
}
end

ASAN report:

ASAN:DEADLYSIGNAL
=================================================================
==31258==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x0001006b06d6 bp 0x7fff5f6bd030 sp 0x7fff5f6b5120 T0)
==31258==The signal is caused by a READ memory access.
==31258==Hint: address points to the zero page.
    #0 0x1006b06d5 in mrb_vm_exec vm.c:1392
    #1 0x1006a39f3 in mrb_vm_run vm.c:862
    #2 0x1006d7cdf in mrb_top_run vm.c:2764
    #3 0x1007a9dd5 in mrb_load_exec parse.y:5780
    #4 0x1007aa725 in mrb_load_file_cxt parse.y:5789
    #5 0x100542e56 in main mruby.c:227
    #6 0x7fffe5638234 in start (libdyld.dylib:x86_64+0x5234)

==31258==Register values:
rax = 0x000060400000efd0  rbx = 0xf2f2f200f2f20000  rcx = 0x001e76f0004b9ca0  rdx = 0x000062f00000c3f8
rdi = 0x0000100000000000  rsi = 0x0003dede00097394  rbp = 0x00007fff5f6bd030  rsp = 0x00007fff5f6b5120
 r8 = 0x0000100000000000   r9 = 0xf6e03d9b8bc60000  r10 = 0x0000100000000000  r11 = 0x00001e2f5f69fa00
r12 = 0xf2f20000f1f1f1f1  r13 = 0x00001fffebed7848  r14 = 0xf2f20000f2f2f2f2  r15 = 0xf2f2f2f2f2040000
AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV vm.c:1392 in mrb_vm_exec
==31258==ABORTING
Abort trap: 6

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