Avoid an redundant copy in pp_flop

Avoid an redundant copy in pp_flop

This copy, which occurs with "a".."z" in list context, has been there
since alphabetic ranges were added in commit b1248f16c (perl 3.0 patch
#17 patch #16, continued).

As a side effect, this:

$ ./miniperl -lwe '()=my $undef1..my $undef2'
Use of uninitialized value in range (or flop) at -e line 1.
Use of uninitialized value in range (or flop) at -e line 1.

becomes this:

$ ./miniperl -lwe '()=my $undef1..my $undef2'
Use of uninitialized value $undef2 in range (or flop) at -e line 1.
Use of uninitialized value in range (or flop) at -e line 1.

which is slightly better. :-)