libpq: Trace StartupMessage/SSLRequest/GSSENCRequest correctly

While working on my patchset for protocol changes I realized that the
StartupMessage/SSLRequest/GSSENCRequest was not shown correctly in the
tracing output of libpq. With this change these messages are now shown
correctly in the tracing output.

To test you can add a PQreset(conn) call to the start of the
test_cancel function in
src/test/modules/libpq_pipeline/libpq_pipeline.c.

And then run:
ninja -C build all install-quiet &&
build/src/test/modules/libpq_pipeline/libpq_pipeline cancel
'port=5432' -t test.trace

And then look at the top of test.trace