ext/pdo_pgsql: Fix the connection state left behind by a lazy fetch

[KentarouTakeda]

With ATTR_EMULATE_PREPARES or ATTR_DISABLE_PREPARES, a lazy fetch (PDO::ATTR_PREFETCH => 0) left the connection busy once it ended, so the next one failed with SQLSTATE[HY000]: another command is already in progress. The cleanup drains the rest of the result set only while the statement is flagged as streaming, and both callers cleared that flag before calling it; let the cleanup clear it itself.

Such a statement also never cleared the connection's pointer to itself, so destroying it left the connection pointing at freed memory for the next lazy fetch to read. Clear the pointer when the statement is destroyed, unless the connection object is gone too.

Independently of how it was prepared, a statement whose stream was taken over by another kept its row counters after its result was freed, so fetch() returned a row of NULLs instead of false. Check the result before trusting the counters.

[KentarouTakeda]

Converting to draft: the drain loop this restores can hang when the statement left the connection in a COPY state. I will update once that is sorted out.