Fix minor memory leak in Standby startup

StandbyRecoverPreparedTransactions() leaked the buffer
used for two phase state file. This was leaked once
at startup and at every shutdown checkpoint seen.

Backpatch to 9.6

Stas Kelvich

Author: simonat2ndQuadrant

src/backend/access/transam/twophase.c

@@ -1886,6 +1886,8 @@ StandbyRecoverPreparedTransactions(bool overwriteOK)
 				Assert(TransactionIdFollows(subxid, xid));
 				SubTransSetParent(xid, subxid, overwriteOK);
 			}
+
+			pfree(buf);
 		}
 	}
 	FreeDir(cldir);