Fix uninitialized memory error.

Fix uninitialized memory error.

It was reported that valgrind found several errors including an
uninitialized memory error in read_startup_packet. It allocates memory
for user name in a startup packet in case cancel or SSL request using
palloc, and later on the memory is used by pstrdup. Since memory
allocated by palloc is undefined, this should have been palloc0.

Bug reported by: Emond Papegaaij
Backpatch-through: v4.1
Discussion:
[pgpool-general: 9065] Re: Segmentation after switchover
https://www.pgpool.net/pipermail/pgpool-general/2024-April/009126.html