Some stylistic improvements in toast_save_datum()

Some stylistic improvements in toast_save_datum()

Move some variables to a smaller scope.  Initialize chunk_data before
storing a pointer to it; this avoids compiler warnings on clang-21, or
respectively us having to work around it by initializing it to zero
before the variable is used (as was done in commit e92677e8633).

Discussion: https://www.postgresql.org/message-id/flat/6604ad6e-5934-43ac-8590-15113d6ae4b1%40eisentraut.org