Fix severe memory leaks in GSSAPI encryption support.

Both the backend and libpq leaked buffers containing encrypted data
to be transmitted, so that the process size would grow roughly as
the total amount of data sent.

There were also far-less-critical leaks of the same sort in GSSAPI
session establishment.

Oversight in commit b0b39f72b, which I failed to notice while
reviewing the code in 2c0cdc818.

Per complaint from pmc@citylink.
Back-patch to v12 where this code was introduced.

Discussion: https://postgr.es/m/20200504115649.GA77072@gate.oper.dinoex.org
