pgsql: Fix compilation warning in xloginsert.c

Fix compilation warning in xloginsert.c

This is reproducible with gcc using at least -O0. The last checks
validating the compression of a block could not be reached with this
variable not set, but let's be clean.

Oversight in 4035cd5, per buildfarm member lapwing.

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/47f514dd9a0022a04d321b627b2e991cb85396e2

Modified Files
--------------
src/backend/access/transam/xloginsert.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)