toke.c: S_scan_heredoc: prune dead code

toke.c: S_scan_heredoc: prune dead code

This incorrect code (using a pointer after finding it to be null)
is the result of the refactoring in 60f40a3895.  It was trying to
account for a string eval with no line break in it.  But that can’t
happen as of 11076590 (if it could it would crash).

So remove it and add an assertion, along with a comment explaining the
assertion.