Fix unchecked return value from strdup

Fix unchecked return value from strdup

The pg_dump compression was using strdup() instead of pg_strdup()
and failed to check the returned pointer for out-of-memory before
dereferencing it. Fix by using pg_strdup() instead which probably
was the intention here in the original patch.

Backpatch to v16 where pg_dump compression was introduced.

Reviewed-by: Tristan Partin 
Reviewed-by: Nathan Bossart 
Discussion: https://postgr.es/m/CC661D60-6F4C-474D-B9CF-E789ACA3CEFC@yesql.se
Backpatch-through: 16