pgsql: Clean up assorted misuses of snprintf()'s result value.

Clean up assorted misuses of snprintf()'s result value.

Fix a small number of places that were testing the result of snprintf()
but doing so incorrectly.  The right test for buffer overrun, per C99,
is "result >= bufsize" not "result > bufsize".  Some places were also
checking for failure with "result == -1", but the standard only says
that a negative value is delivered on failure.

(Note that this only makes these places correct if snprintf() delivers
C99-compliant results.  But at least now these places are consistent
with all the other places where we assume that.)

Also, make psql_start_test() and isolation_start_test() check for
buffer overrun while constructing their shell commands.  There seems
like a higher risk of overrun, with more severe consequences, here
than there is for the individual file paths that are made elsewhere
in the same functions, so this seemed like a worthwhile change.

Also fix guc.c's do_serialize() to initialize errno = 0 before
calling vsnprintf.  In principle, this should be unnecessary because
vsnprintf should have set errno if it returns a failure indication ...
but the other two places this coding pattern is cribbed from don't
assume that, so let's be consistent.

These errors are all very old, so back-patch as appropriate.  I think
that only the shell command overrun cases are even theoretically
reachable in practice, but there's not much point in erroneous error
checks.

Discussion: https://postgr.es/m/17245.1534289329@sss.pgh.pa.us

Branch
------
REL9_4_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d371efb39c33f79ad5f6741d76bfae54df21eb55

Modified Files
--------------
src/backend/libpq/ip.c                  |  6 +++---
src/backend/postmaster/pgstat.c         |  2 +-
src/interfaces/ecpg/pgtypeslib/common.c |  2 +-
src/port/getaddrinfo.c                  |  2 +-
src/test/isolation/isolation_main.c     | 24 ++++++++++++++++++------
src/test/regress/pg_regress.c           |  2 +-
src/test/regress/pg_regress_main.c      | 26 +++++++++++++++++++-------
7 files changed, 44 insertions(+), 20 deletions(-)