c3 bug with ISA[i] = undef

[rurban]

t/mro/basic.t test 26 fails with c3:

    {
        package ISACLEAR;
        our @ISA = qw/XX YY ZZ/;
    }
    # baseline
    ok(eq_array(mro::get_linear_isa('ISACLEAR'),[qw/ISACLEAR XX YY ZZ/]));

    # this looks dumb, but it preserves existing behavior for compatibility
    #  (undefined @ISA elements treated as "main")
    # This revealed a c3 bug (undetected since 2007)
    $ISACLEAR::ISA[1] = undef;
    ok(eq_array(mro::get_linear_isa('ISACLEAR'),[qw/ISACLEAR XX main ZZ/]))
      or diag "'".join("' '",@{mro::get_linear_isa('ISACLEAR')})."'";

=>
'ISACLEAR' 'XX' 'main' 'ZZ' with dfs
'ISACLEAR' 'XX' 'main' '' 'ZZ' with c3.

Note the empty entry after the inserted/changed main. YY => main.
This problem existed since 2007, the initial c3 implementation. It was never tested.

Detected by #209

[rurban]

This might be change artefact from #209, but old c3 mro had it before also:

old (dfs as default):
$ cperl5.25.3d-nt -Mmro=c3 -e'@ISACLEAR::ISA = qw/XX YY ZZ/; mro::set_mro("ISACLEAR","c3");
  $ISACLEAR::ISA[1] = undef; print join",",@{mro::get_linear_isa('ISACLEAR')}'
ISACLEAR,XX,main,,ZZ
new (c3 as default):
$ cperl -Mmro -e'@ISACLEAR::ISA = qw/XX YY ZZ/; 
  $ISACLEAR::ISA[1] = undef; print join",",@{mro::get_linear_isa('ISACLEAR')}'
ISACLEAR,XX,main,,ZZ

It's not ext/mro. The deleted ISA entry itself is replaced by NULL everywhere:

$ cperl -Mmro=dfs -e'@ISACLEAR::ISA=qw/XX YY ZZ/;mro::set_mro("ISACLEAR","dfs");$ISACLEAR::ISA[1]=undef; 
    print join",",@ISACLEAR::ISA'
XX,,ZZ
$ cperl -Mmro=c3 -e'@ISACLEAR::ISA=qw/XX YY ZZ/;mro::set_mro("ISACLEAR","c3");$ISACLEAR::ISA[1]=undef; 
    print join",",@ISACLEAR::ISA'
XX,,ZZ
$ cperl -e'@ISACLEAR::ISA=qw/XX YY ZZ/;mro::set_mro("ISACLEAR","c3");$ISACLEAR::ISA[1]=undef;
    print join",",@ISACLEAR::ISA'
XX,,ZZ

but mro_get_linear_isa_c3 needs to replace NULL with main, or as in this case skip it.