Avoid read-after-free in S_scan_heredoc() if the terminator line has no "\n".

The code added by commit 112d128413206514 to fix RT #65838 (Allow here-doc
with no final newline) could in some rare cases cause a read of free()d
memory during parsing. The code itself is only run if the Perl program
ends with a heredoc (which is an unusual structure), and if the last line of
the file on disk has no terminating newline character (which is also unusual,
as many editors default to adding a final newline). The bug would be
triggered if the fixup code in S_scan_heredoc() triggered a reallocation of
the buffer in PL_linestr when adding a newline to it.