heap-buffer-overflow (read outside of buffer) in mrb_vm_exec()

Triggered in `3231219` (14 April 2017). Compiled with afl-gcc + asan. `./mirb < test000` ``` ==10555==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x60200001c920 at pc 0x52be2c bp 0x7ffe6751ada0 sp 0x7ffe6751ad98 READ of size 16 at 0x60200001c920 thread T0 #0 0x52be2b in mrb_vm_exec /root/mruby/src/vm.c:1556 #1 0x530a19 in mrb_vm_run /root/mruby/src/vm.c:829 #2 0x53260f in mrb_run /root/mruby/src/vm.c:2644 #3 0x53260f in ecall /root/mruby/src/vm.c:320 #4 0x4fd59f in mrb_vm_exec /root/mruby/src/vm.c:1716 #5 0x530a19 in mrb_vm_run /root/mruby/src/vm.c:829 #6 0x53260f in mrb_run /root/mruby/src/vm.c:2644 #7 0x53260f in ecall /root/mruby/src/vm.c:320 #8 0x508b65 in mrb_vm_exec /root/mruby/src/vm.c:1170 #9 0x530a19 in mrb_vm_run /root/mruby/src/vm.c:829 #10 0x53260f in mrb_run /root/mruby/src/vm.c:2644 #11 0x53260f in ecall /root/mruby/src/vm.c:320 #12 0x508b65 in mrb_vm_exec /root/mruby/src/vm.c:1170 #13 0x530a19 in mrb_vm_run /root/mruby/src/vm.c:829 #14 0x40781f in main /root/mruby/mrbgems/mruby-bin-mirb/tools/mirb/mirb.c:549 #15 0x7f93c4ea5b44 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b44) #16 0x40a71c (/root/mruby/bin/mirb+0x40a71c) 0x60200001c920 is located 0 bytes to the right of 16-byte region [0x60200001c910,0x60200001c920) allocated by thread T0 here: #0 0x7f93c55849f6 in __interceptor_realloc (/usr/lib/x86_64-linux-gnu/libasan.so.1+0x549f6) #1 0x421a2e in mrb_realloc_simple /root/mruby/src/gc.c:202 #2 0x421a2e in mrb_realloc /root/mruby/src/gc.c:216 #3 0x421a2e in mrb_malloc /root/mruby/src/gc.c:237 SUMMARY: AddressSanitizer: heap-buffer-overflow /root/mruby/src/vm.c:1556 mrb_vm_exec ```