Fix SIGSEGV while catchup without passing --pgdata/-D or PGDATA varia…

[ZigzagAK]

pg_probackup crashes on catchup.

       Message: Process 846075 (pg_probackup) of user 60240 dumped core.                Stack trace of thread 846075:
                #0  0x000000000044c930 strlcpy (pg_probackup)
                #1  0x000000000044cc05 join_path_components (pg_probackup)
                #2  0x000000000042cfbe slurpFile (pg_probackup)
                #3  0x000000000043e64a set_min_recovery_point (pg_probackup)
                #4  0x0000000000441207 do_catchup (pg_probackup)
                #5  0x0000000000405912 main (pg_probackup)
                #6  0x00007f7b2919a7e5 __libc_start_main (libc.so.6)
                #7  0x0000000000405e6e _start (pg_probackup) 

pg_probackup  catchup -b FULL \
  --source-pgdata=/pg_data/17 \
  --destination-pgdata=/pg_data/test/17 \
  --stream --temp-slot \
  --threads=4 \
  --exclude-path=postgresql.conf \
  --exclude-path=postgresql.auto.conf \
  --exclude-path=pg_hba.conf \
  --exclude-path=pg_ident.conf \
  --remote-host=msk-itc-pga1 \
  --remote-user=postgres \
  --remote-path=/opt/pg17/bin \
  --log-directory=/pg_data/_catchup \
  --log-level-console=verbose \
  --log-format-console=plain \
  --log-level-file=verbose \
  --log-format-file=plain \
  --log-filename=catchup_%Y-%m-%d.%H:%M:%S.log \
  --error-log-filename=catchup_%Y-%m-%d.%H:%M:%S.err \
  -d backup_pgdb \
  -U backup \
  -w