Use of uninitialized memory in mrb_ptr_to_str

[clayton-shopify]

The following input demonstrates use of uninitialized memory:

[].methods.sort do |x|
  x.to_proc[Symbol,Kernel].Z
rescue
end.inspect

Valgrind shows the following error:

==5582== Conditional jump or move depends on uninitialised value(s)
==5582==    at 0x409CAF: mrb_ptr_to_str (string.c:1689)
==5582==    by 0x422F15: mrb_any_to_s (object.c:438)
==5582==    by 0x43B853: mrb_obj_inspect (kernel.c:59)
==5582==    by 0x434383: mrb_vm_exec (vm.c:1470)
==5582==    by 0x4392BB: mrb_vm_run (vm.c:978)
==5582==    by 0x43AF78: mrb_top_run (vm.c:3018)
==5582==    by 0x44B86E: mrb_load_exec (parse.y:6139)
==5582==    by 0x44D296: mrb_load_file_cxt (parse.y:6148)
==5582==    by 0x402ED5: main (mruby.c:280)

This issue was reported by Denis Kasak & Dinko Galetić (https://hackerone.com/dgaletic).