Fix $lex = chomp $lex regression from 5.12

Commit 2f9970be0 (5.13.9) caused the lhs to be written to with 0
before the chomp happens, and then incremented for each chomp on the
rhs argument.

That makes

    my $a = $/ = 7;
    $a = chomp $a;
    print $a;

print 0 instead of 1 and makes

    my $a = $/ = 0;
    $a = chomp $a;

assign the empty string to $a instead of 1.