BUG #19092: scram_free() will free on address which was not malloc()-ed in pg_scram_mech

The following bug has been logged on the website:

Bug reference:      19092
Logged by:          NJUEEXRM
Email address:      13952878799@163.com
PostgreSQL version: 17.0
Operating system:   MacOS 12.7.6
Description:        

The issue is about the only implementation of pg_fe_sasl_mech interface:
pg_scram_mech. In the init func of pg_scram_mech, the variable
state->password is assigned by variable prep_password, which is prepared in
function pg_saslprep(). However, pg_saslprep() will use palloc/pfree or
malloc/free determined by FRONTEND marco。If we are in backend env，the
prep_password will be palloc-ed in CurrentMemoryContext. The problem is
state->password will be released by free() in the free func of
pg_scram_mech: scram_free, and will cause 'free on address which was not
malloc()-ed' error.
This issue occurred when I was attempting to make a connection to Backend
via libpq interfaces in Backend itself.