pg_stat_statements: Set PlannedStmt to NULL after nested utility execution

pg_stat_statements: Set PlannedStmt to NULL after nested utility execution

As mentioned in 8268e41aca23, pgss_ProcessUtility() may free the
PlannedStmt after an internal ROLLBACK.  This commit sets the
PlannedStmt "pstmt" to NULL once it is no longer safe to rely on it,
making bugs similar to the one fixed by the previous commit easier to
detect.

Suggested-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/0A9A8DAC-BC3C-4C7A-9504-2C6050405544@anarazel.de