Null pointer dereference in OP_SEND

[clayton-shopify]

The following input demonstrates a crash:

def method_missing(m)
ensure
begin A rescue
break
rescue
end
end

send ''

ASAN report:

ASAN:DEADLYSIGNAL
=================================================================
==3315==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x00010b264e0a bp 0x7fff54b08f90 sp 0x7fff54b00d60 T0)
==3315==The signal is caused by a READ memory access.
==3315==Hint: address points to the zero page.
    #0 0x10b264e09 in mrb_vm_exec (mruby:x86_64+0x10016fe09)
    #1 0x10b258944 in mrb_vm_run (mruby:x86_64+0x100163944)
    #2 0x10b28e41f in mrb_top_run (mruby:x86_64+0x10019941f)
    #3 0x10b36724d in mrb_load_exec (mruby:x86_64+0x10027224d)
    #4 0x10b368065 in mrb_load_file_cxt (mruby:x86_64+0x100273065)
    #5 0x10b0f75a3 in main mruby.c:227
    #6 0x7fffe5638234 in start (libdyld.dylib:x86_64+0x5234)

==3315==Register values:
rax = 0x000000010b4165e0  rbx = 0xf2f20000f2f2f200  rcx = 0x0000000000000000  rdx = 0x0000630000001e00
rdi = 0x0000100000000000  rsi = 0x0000630000001e70  rbp = 0x00007fff54b08f90  rsp = 0x00007fff54b00d60
 r8 = 0x0000000000000000   r9 = 0x000000000000000e  r10 = 0x0000000000000000  r11 = 0xfffffe7000001d50
r12 = 0xf2f20000f1f1f1f1  r13 = 0x00001fffea961030  r14 = 0xf2f20000f2f2f2f2  r15 = 0xf2f2f2f2f2040000
AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV (mruby:x86_64+0x10016fe09) in mrb_vm_exec
==3315==ABORTING
Abort trap: 6

This issue was reported by @titanous