lex_grow_linestr(): update re_eval_start

lex_grow_linestr(): update re_eval_start

As seen in [perl #114242], the TryCatch test suite triggers this bug.
lex_grow_linestr() was reallocing the PL_linestr buffer and updating all
pointers within it, but was failing to update
PL_sublex_info.re_eval_start.

Unfortunately, lex_grow_linestr() doesn't seem to be used in core outside
of hereto docs, and due to a separate parser bug, hereto docs can't
currently be used within /(?{})/, so I can't add tests for it yet.