Fix psql_scan crash.

Fix psql_scan crash.

It was reported that psql_scan crashes while determining whether a
string in a long query is psql variable (i.e. starting with ":") or
not.

https://github.com/pgpool/pgpool2/issues/54

This is because callback struct were not provided while calling
psql_scan_create().  Later psql_scan() tries to invoke a callback and
crashes because the pointer to the callback struct is NULL. To fix
this, provide PsqlScanCallbacks struct with a NULL pointer inside to
the callback function. With this, psql_scan() avoids to invoke a
callback.

Backpatch to master, V4.5, V4.4, V4.3, V4.2 and V4.1 where psql_scan
was introduced.