psql: Fix some scan-build warnings

psql: Fix some scan-build warnings

A repeated complaint was that scan-build thought that if the \timing
setting changes during processing of a query, the post-processing
might read garbage time values.  This is probably not possible right
now, but it's not entirely inconceivable given the code structure.  So
silence this warning with small restructuring that makes this more
robust.  The other warnings were a few dead stores that are easy to
remove.

Discussion: https://www.postgresql.org/message-id/2570e2ae-fa0f-aac9-f72f-bb59a9983a20@enterprisedb.com