Null pointer dereference in mrb_str_to_inum

[clayton-shopify]

The following input demonstrates a crash:

def method_missing(*)
  false
end

def to_str()
  ""
end

Integer(a,2)

ASAN report:

==56841==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000001 (pc 0x00010c93c7d6 bp 0x7fff533c9030 sp 0x7fff533c8e60 T0)
==56841==The signal is caused by a READ memory access.
==56841==Hint: address points to the zero page.
    #0 0x10c93c7d5 in mrb_str_to_inum string.c:2257
    #1 0x10c9100ea in mrb_convert_to_integer object.c:548
    #2 0x10ca3b78f in mrb_f_integer (mruby:x86_64+0x10020e78f)
    #3 0x10c99cd98 in mrb_vm_exec vm.c:1268
    #4 0x10c991f49 in mrb_vm_run vm.c:824
    #5 0x10c9c4859 in mrb_top_run vm.c:2630
    #6 0x10ca95845 in mrb_load_exec parse.y:5762
    #7 0x10ca96655 in mrb_load_file_cxt parse.y:5771
    #8 0x10c82ee76 in main mruby.c:227
    #9 0x7fffbbbba234 in start (libdyld.dylib:x86_64+0x5234)

==56841==Register values:
rax = 0x0000000000000001  rbx = 0x00007fff533c8ec0  rcx = 0x0000000000000001  rdx = 0x0000100000000000
rdi = 0x0000100000000000  rsi = 0x0000100000000000  rbp = 0x00007fff533c9030  rsp = 0x00007fff533c8e60
 r8 = 0x000062f000000010   r9 = 0x0000100000000000  r10 = 0x26cf1d925a730000  r11 = 0xffffffffffffffc0
r12 = 0xf2f20000f1f1f1f1  r13 = 0x00001fffea67a084  r14 = 0x00007fff533c90c0  r15 = 0x00007fff533c90e0
AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV string.c:2257 in mrb_str_to_inum
==56841==ABORTING
Abort trap: 6

This issue was reported by https://hackerone.com/ston3