Repair various defects in dc212340058b4e7ecfc5a7a81ec50e7a207bf288.

Repair various defects in dc212340058b4e7ecfc5a7a81ec50e7a207bf288.

pg_combinebackup had various problems:

* strncpy was used in various places where strlcpy should be used
  instead, to avoid any possibility of the result not being
  \0-terminated.
* scan_for_existing_tablespaces() failed to close the directory,
  and an error when opening the directory was reported with the
  wrong pathname.
* write_reconstructed_file() contained some redundant and therefore
  dead code.
* flush_manifest() didn't check the result of pg_checksum_update()
  as we do in other places, and misused a local pathname variable
  that shouldn't exist at all.

In pg_basebackup, the wrong variable name was used in one place,
due to a copy and paste that was not properly adjusted.

In blkreftable.c, the loop incorrectly doubled chunkno instead of
max_chunks. Fix that. Also remove a nearby assertion per repeated
off-list complaints from Tom Lane.

Per Coverity and subsequent code inspection by me and by Tom Lane.

Discussion: http://postgr.es/m/CA+Tgmobvqqj-DW9F7uUzT-cQqs6wcVb-Xhs=w=hzJnXSE-kRGw@mail.gmail.com