perl -Dr: avoid coredump in \1

When displaying each reg node being executed, the code that dumps a REF
node assumed that a capture was valid if progs->offs[n].start != -1.
In fact during backtracking after a failure, a capture is "undone" by
merely setting progs->offs[n].end = -1.

So make the dump code account for that too.

This was causing a test in t/re/pat.t to coredump:

    use re qw(Debug EXECUTE);
    "x" =~ m{ () y | () \1 }x;

Although given that neither the test nor the REF code in regprop() have
changed recently, I'm not sure why this has only recently started crashing.

0 files changed, 0 insertions, 0 deletions