logtape.c: allocate read buffer even for an empty tape.

Prior to this commit, the read buffer was allocated at the time the tape
was rewound; but as an optimization, would not be allocated at all if
the tape was empty.

That optimization meant that it was valid to have a rewound tape with
the buffer set to NULL, but only if a number of conditions were met
and only if the API was used properly. After 7fdd919a refactored the
code to support lazily-allocating the buffer, Coverity started
complaining.

The optimization for empty tapes doesn't seem important, so just
allocate the buffer whether the tape has any data or not.

Discussion: https://postgr.es/m/20351.1581868306%40sss.pgh.pa.us

src/backend/utils/sort/logtape.c

@@ -544,11 +544,8 @@ ltsConcatWorkerTapes(LogicalTapeSet *lts, TapeShare *shared,
 static void
 ltsInitReadBuffer(LogicalTapeSet *lts, LogicalTape *lt)
 {
-	if (lt->firstBlockNumber != -1L)
-	{
 	Assert(lt->buffer_size > 0);
 	lt->buffer = palloc(lt->buffer_size);
-	}
 
 	/* Read the first block, or reset if tape is empty */
 	lt->nextBlockNumber = lt->firstBlockNumber;
@@ -839,13 +836,10 @@ LogicalTapeRewindForRead(LogicalTapeSet *lts, int tapenum, size_t buffer_size)
 	/* Allocate a read buffer (unless the tape is empty) */
 	if (lt->buffer)
 		pfree(lt->buffer);
-	lt->buffer = NULL;
-	lt->buffer_size = 0;
-	if (lt->firstBlockNumber != -1L)
-	{
+
 	/* the buffer is lazily allocated, but set the size here */
+	lt->buffer = NULL;
 	lt->buffer_size = buffer_size;
-	}
 }
 
 /*