[perl #121259] Always allow COW after $input=<>

sv_grow provides an extra byte for the sake of copy-on-write’s buffer
reference count, but skips this for multiples of 256 (though the com-
ments there say powers of two).

When we do

    $input=<>

The assignment is optimised away and the allocation takes place in
sv_gets.  In that code path, we know we don’t need a nice power of
two and allocating an extra byte won’t hurt, so go ahead and add an
extra byte.

This speeds up code doing m//g on $input, because it avoids the pre-
match copy.