Fix inconsistency in determining the timestamp of the db statfile.

We use the timestamp of the global statfile if we are not able to
determine it for a particular database in case the entry for that database
doesn't exist. However, we were using it even when the statfile is
corrupt.

As there is no user reported issue and it is not clear if there is any
impact of this on actual application so decided not to backpatch.

Reported-by: Amit Kapila
Author: Amit Kapila
Reviewed-by: Sawada Masahiko, Magnus Hagander and Alvaro Herrera
Discussion: https://postgr.es/m/CAA4eK1J3oTJKyVq6v7K4d3jD+vtnruG9fHRib6UuWWsrwAR6Aw@mail.gmail.com

src/backend/postmaster/pgstat.c

@@ -5557,7 +5557,8 @@ done:
  * pgstat_read_db_statsfile_timestamp() -
  *
  *	Attempt to determine the timestamp of the last db statfile write.
- *	Returns true if successful; the timestamp is stored in *ts.
+ *	Returns true if successful; the timestamp is stored in *ts. The caller must
+ *	rely on timestamp stored in *ts iff the function returns true.
  *
  *	This needs to be careful about handling databases for which no stats file
  *	exists, such as databases without a stat entry or those not yet written:
@@ -5665,7 +5666,8 @@ pgstat_read_db_statsfile_timestamp(Oid databaseid, bool permanent,
 					ereport(pgStatRunningInCollector ? LOG : WARNING,
 							(errmsg("corrupted statistics file \"%s\"",
 									statfile)));
-					goto done;
+					FreeFile(fpin);
+					return false;
 				}
 
 				/*
@@ -5684,10 +5686,13 @@ pgstat_read_db_statsfile_timestamp(Oid databaseid, bool permanent,
 				goto done;
 
 			default:
-				ereport(pgStatRunningInCollector ? LOG : WARNING,
-						(errmsg("corrupted statistics file \"%s\"",
-								statfile)));
-				goto done;
+				{
+					ereport(pgStatRunningInCollector ? LOG : WARNING,
+							(errmsg("corrupted statistics file \"%s\"",
+									statfile)));
+					FreeFile(fpin);
+					return false;
+				}
 		}
 	}