Using groups[0].e turns out to be unreliable to print the suffix of the
subject string for a regsub operation. On Debian buster, with the help
of ASAN we can observe uninitialized memory through the remains of ASAN's
0xbe pattern that leads later to a complaint about an invalid pointer:

    runtime error: pointer index expression with base 0x6310000a0816
    overflowed to 0xbebf21cebec8c6d4

With a simple subtraction we can confirm the offset added to the base
address:

    0xbebf21cebec8c6d4 - 0x6310000a0816 = 0xbebebebebebebebe

To work around the possibility of an uninitialized ovector depending on
the pcre2 version, we initialize all offsets to PCRE2_UNSET and when we
encounter that value we capture a safe empty token.

This means that at the end of VRE_sub() we can no longer count on the
capture of the 0th group and revert back to using the offset.