Repair various defects in dc21234005.

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

src/bin/pg_combinebackup/reconstruct.c

@@ -577,13 +577,12 @@ write_reconstructed_file(char *input_filename,
 			{
 				if (current_block == start_of_range)
 					appendStringInfo(&debug_buf, " %u:%s@" UINT64_FORMAT,
-									 current_block,
-									 s == NULL ? "ZERO" : s->filename,
+									 current_block, s->filename,
 									 (uint64) offsetmap[current_block]);
 				else
 					appendStringInfo(&debug_buf, " %u-%u:%s@" UINT64_FORMAT,
 									 start_of_range, current_block,
-									 s == NULL ? "ZERO" : s->filename,
+									 s->filename,
 									 (uint64) offsetmap[current_block]);
 			}