Ruby -- Double free in Regexp compilation

piao reports:

    Due to a bug in the Regexp compilation process, creating
    a Regexp object with a crafted source string could cause
    the same memory to be freed twice. This is known as a
    "double free" vulnerability. Note that, in general, it
    is considered unsafe to create and use a Regexp object
    generated from untrusted input. In this case, however,
    following a comprehensive assessment, we treat this issue
    as a vulnerability.