Null pointer dereference in ecall

[clayton-shopify]

The following input demonstrates a crash:

Fiber.new {
    begin
        next
        ensure -> {}
    end
}.resume

This appears to be closely related to #3662.

ASAN report:

ASAN:DEADLYSIGNAL
=================================================================
==17700==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x00010793e8ea bp 0x7fff5844b1d0 sp 0x7fff5844ae20 T0)
==17700==The signal is caused by a READ memory access.
==17700==Hint: address points to the zero page.
    #0 0x10793e8e9 in ecall vm.c:310
    #1 0x107926168 in mrb_vm_exec vm.c:1905
    #2 0x10790e14f in mrb_vm_run vm.c:860
    #3 0x107941eff in mrb_top_run vm.c:2750
    #4 0x107a13fb5 in mrb_load_exec parse.y:5780
    #5 0x107a14905 in mrb_load_file_cxt parse.y:5789
    #6 0x1077ad7c6 in main mruby.c:227
    #7 0x7fff8c15e234 in start (libdyld.dylib:x86_64+0x5234)

==17700==Register values:
rax = 0x0000000000000000  rbx = 0x00007fff5844ae80  rcx = 0x000060b00000aa58  rdx = 0x0000100000000000
rdi = 0x0000000000000050  rsi = 0x0000100000000000  rbp = 0x00007fff5844b1d0  rsp = 0x00007fff5844ae20
 r8 = 0x00007fff5844ae60   r9 = 0x0000000000000000  r10 = 0x00001c280000148c  r11 = 0x0000100000000000
r12 = 0xf2f20000f1f1f1f1  r13 = 0x00001fffeb08a450  r14 = 0xf2f20000f2f2f2f2  r15 = 0xf2f2f2f2f2040000
AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV vm.c:310 in ecall
==17700==ABORTING
Abort trap: 6

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