pgsql: Remember to reset yy_start state when firing up repl_scanner.l.

Remember to reset yy_start state when firing up repl_scanner.l.

Without this, we get odd behavior when the previous cycle of
lexing exited in a non-default exclusive state.  Every other
copy of this code is aware that it has to do BEGIN(INITIAL),
but repl_scanner.l did not get that memo.

The real-world impact of this is probably limited, since most
replication clients would abandon their connection after getting
a syntax error.  Still, it's a bug.

This mistake is old, so back-patch to all supported branches.

Discussion: https://postgr.es/m/1874781.1643035952@sss.pgh.pa.us

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/449a696236ff6d8a86b98ae6503aa51dc39a37b2

Modified Files
--------------
src/backend/replication/repl_scanner.l | 3 +++
1 file changed, 3 insertions(+)