Fix strsep() use for SCRAM secrets parsing

Fix strsep() use for SCRAM secrets parsing

The previous code (from commit 5d2e1cc117b) did not detect end of
string correctly, so it would fail to error out if fewer than the
expected number of fields were present, which could then later lead to
a crash when NULL string pointers are accessed.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Reported-by: Ranier Vilela <ranier.vf@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/79692bf9-17d3-41e6-b9c9-fc8c3944222a@eisentraut.org