Null pointer dereference with send/method_missing

The following program triggers a null pointer dereference with mruby b200c747: ```ruby def method_missing(m) ensure begin A rescue break rescue end end send '' ``` ASAN report: ```text ASAN:DEADLYSIGNAL ================================================================= ==12116==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x00000051bfaa bp 0x7fff4a650cd0 sp 0x7fff4a648a80 T0) #0 0x51bfa9 in mrb_vm_exec /home/vagrant/mruby/src/vm.c:1427:9 #1 0x510c6a in mrb_vm_run /home/vagrant/mruby/src/vm.c:879:12 #2 0x541b3f in mrb_top_run /home/vagrant/mruby/src/vm.c:2884:12 #3 0x6569ff in mrb_load_exec /home/vagrant/mruby/mrbgems/mruby-compiler/core/parse.y:5823:7 #4 0x657685 in mrb_load_file_cxt /home/vagrant/mruby/mrbgems/mruby-compiler/core/parse.y:5832:10 #5 0x4f3a61 in main /home/vagrant/mruby/mrbgems/mruby-bin-mruby/tools/mruby/mruby.c:227:11 #6 0x7f256672ef44 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21f44) #7 0x41a5c5 in _start (/home/vagrant/mruby/bin/mruby+0x41a5c5) AddressSanitizer can not provide additional info. SUMMARY: AddressSanitizer: SEGV /home/vagrant/mruby/src/vm.c:1427:9 in mrb_vm_exec ==12116==ABORTING ```