Minor code review for tuple slot rewrite.

Avoid creating transiently-inconsistent slot states where possible,
by not setting TTS_FLAG_SHOULDFREE until after the slot actually has
a free'able tuple pointer, and by making sure that we reset tts_nvalid
and related derived state before we replace the tuple contents.  This
would only matter if something were to examine the slot after we'd
suffered some kind of error (e.g. out of memory) while manipulating
the slot.  We typically don't do that, so these changes might just be
cosmetic --- but even if so, it seems like good future-proofing.

Also remove some redundant Asserts, and add a couple for consistency.

Back-patch to v12 where all this code was rewritten.

Discussion: https://postgr.es/m/16095-c3ff2e5283b8dba5@postgresql.org

src/include/executor/tuptable.h

@@ -261,9 +261,8 @@ typedef struct BufferHeapTupleTableSlot
 	/*
 	 * If buffer is not InvalidBuffer, then the slot is holding a pin on the
 	 * indicated buffer page; drop the pin when we release the slot's
-	 * reference to that buffer.  (TTS_FLAG_SHOULDFREE should not be set be
-	 * false in such a case, since presumably tts_tuple is pointing at the
-	 * buffer page.)
+	 * reference to that buffer.  (TTS_FLAG_SHOULDFREE should not be set in
+	 * such a case, since presumably tts_tuple is pointing into the buffer.)
 	 */
 	Buffer		buffer;			/* tuple's buffer, or InvalidBuffer */
 } BufferHeapTupleTableSlot;