pgsql: Use snprintf not sprintf in pg_waldump's timestamptz_to_str.

Use snprintf not sprintf in pg_waldump's timestamptz_to_str.

This could only cause an issue if strftime returned a ridiculously
long timezone name, which seems unlikely; and it wouldn't qualify
as a security problem even then, since pg_waldump (nee pg_xlogdump)
is a debug tool not part of the server.  But gcc 8 has started issuing
warnings about it, so let's use snprintf and be safe.

Backpatch to 9.3 where this code was added.

Discussion: https://postgr.es/m/21789.1529170195@sss.pgh.pa.us

Branch
------
REL9_4_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/fd079dd0915a2005da2fbfd75fda1cc3611f3a2f

Modified Files
--------------
contrib/pg_xlogdump/compat.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)