Commit 1eb6d652 introduced zeroed alignment bytes in the GID field
of commit/abort WAL records.  Fixup commit cf5a1890 later changed
that representation into a regular cstring with a single terminating
zero byte, but it also introduced an off-by-one mistake.  Fix that.

Author: Nikhil Sontakke
Reported-by: Nikhil Sontakke
Discussion: https://postgr.es/m/CAMGcDxey6dG1DP34_tJMoWPcp5sPJUAL4K5CayUUXLQSx2GQpA@mail.gmail.com