Fix a strict aliasing violation

Fix a strict aliasing violation

This one is almost a textbook example of an aliasing violation, and it
is straightforward to fix, so clean it up.  (The warning only shows up
if you remove the -fno-strict-aliasing option.)  Also, move the code
after the error checking.  Doesn't make a difference technically, but
it seems strange to do actions before errors are checked.

Reported-by: Tatsuo Ishii <ishii@postgresql.org>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/20240724.155525.366150353176322967.ishii%40postgresql.org