pg_stat_statements: Fix handling of duplicate constant locations

Two or more constants can have the same location.  We handled this
correctly for non squashed constants, but failed to do it if squashed
(resulting in out-of-bounds memory access), because the code structure
became broken by commit 0f65f3eec478: we failed to update 'last_loc'
correctly when skipping these squashed constants.

The simplest fix seems to be to get rid of 'last_loc' altogether -- in
hindsight, it's quite pointless.  Also, when ignoring a constant because
of this, make sure to fulfill fill_in_constant_lengths's duty of setting
its length to -1.

Lastly, we can use == instead of <= because the locations have been
sorted beforehand, so the < case cannot arise.

Co-authored-by: Sami Imseih <samimseih@gmail.com>
Co-authored-by: Dmitry Dolgov <9erthalion6@gmail.com>
Reported-by: Konstantin Knizhnik <knizhnik@garret.ru>
Backpatch-through: 18
Discussion: https://www.postgresql.org/message-id/2b91e358-0d99-43f7-be44-d2d4dbce37b3%40garret.ru

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