Fix handling of missing files when using pg_rewind with online source

When working with an online source cluster, pg_rewind gets a list of all
the files in the source data directory using a WITH RECURSIVE query,
returning a NULL result for a file's metadata if it gets removed between
the moment it is listed in a directory and the moment its metadata is
obtained with pg_stat_file() (say a recycled WAL segment).  The query
result was processed in such a way that for each tuple we checked only
that the first file's metadata was NULL.  This could have two
consequences, both resulting in a failure of the rewind:
- If the first tuple referred to a removed file, all files from the
source would be ignored.
- Any file actually missing would not be considered as such.

While on it, rework slightly the code so as no values are saved if we
know that a file is going to be skipped.

Issue introduced by b36805f, so backpatch down to 9.5.

Author: Justin Pryzby, Michael Paquier
Reviewed-by: Daniel Gustafsson, Masahiko Sawada
Discussion: https://postgr.es/m/20200713061010.GC23581@telsasoft.com
Backpatch-through: 9.5

Author: michaelpq

src/bin/pg_rewind/libpq_fetch.c

@@ -214,13 +214,13 @@ libpqProcessFileList(void)
 	/* Read result to local variables */
 	for (i = 0; i < PQntuples(res); i++)
 	{
-		char	   *path = PQgetvalue(res, i, 0);
-		int64		filesize = atol(PQgetvalue(res, i, 1));
-		bool		isdir = (strcmp(PQgetvalue(res, i, 2), "t") == 0);
-		char	   *link_target = PQgetvalue(res, i, 3);
+		char	   *path;
+		int64		filesize;
+		bool		isdir;
+		char	   *link_target;
 		file_type_t type;
 
-		if (PQgetisnull(res, 0, 1))
+		if (PQgetisnull(res, i, 1))
 		{
 			/*
 			 * The file was removed from the server while the query was
@@ -229,6 +229,11 @@ libpqProcessFileList(void)
 			continue;
 		}
 
+		path = PQgetvalue(res, i, 0);
+		filesize = atol(PQgetvalue(res, i, 1));
+		isdir = (strcmp(PQgetvalue(res, i, 2), "t") == 0);
+		link_target = PQgetvalue(res, i, 3);
+
 		if (link_target[0])
 			type = FILE_TYPE_SYMLINK;
 		else if (isdir)