Fix parser buffer corruption with multiline *{...}

Since commit a49b10d0a, it has been possible for scan_ident in toke.c
to reallocate the parser’s buffer (SvPVX(PL_linestr)) when scanning
for multiline whitespace.

For the sake of those cases where it finds an arbitrary expression,
not just an identifier, it records a pointer to the first opening
brace, which it returns to the parser after finding out that there is
indeed an expression.

That pointer was not being updated when the buffer was being
allocated.

The solution is to record an offset, rather than a pointer, of the
opening brace relative to the beginning of the current line of input.

This one-liner:

$ ./miniperl  -e '*{' -e '         XS::APItest::gv_fetchmeth_type()' -e '}'

was giving me:

Unrecognized character \x80; marked by <-- HERE after 2<-- HERE near column 24 at -e line 2.

(There were nine nulls before the 2, but git stripped them out.)
Fix line nums when multiline ${expr} spans here-doc

<<end . ${

end
"bar"};
warn __LINE__ # 3, not 5

This was caused by commit a49b10d0a, which make scan_ident in toke.c
reallocate the parser’s current line buffer (SvPVX(PL_linestr)) to
search for whitespace surrounding an identifier.

In case there is an arbitrary expression, it temporarily records the
line number and resets it at the end if that turns out to be the case.
However, it was not resetting PL_parser->herelines, which records how
many line numbers to skip when next incrementing it (to skip past
here-doc bodies).

So save and restore that value, too.