Heap buffer overflow introduced in #3553

[Asmod4n]

https://github.com/mruby/mruby/pull/3553/files#diff-d13030dbdc98ec484fa8f0e9cbc31443R218 is accessing one element too much of the array, tests still pass when <= is changed to < in for (j = i; j <= a->len - len; ++j) { block but dunno if it still works as expected.

These kind of bugs can be found quite good with

  conf.cc.flags << '-fsanitize=address'
  conf.linker.flags << '-fsanitize=address'

added to build_config.rb