Fix incorrect computations of length of null bitmap in pageinspect.

Instead of using our standard macro for this calculation, this code
did it itself ... and got it wrong, leading to incorrect display of
the null bitmap in some cases.  Noted and fixed by Maksim Milyutin.

In passing, remove a uselessly duplicative error check.

Errors were introduced in commit d6061f8; back-patch to 9.6
where that came in.

Maksim Milyutin, reviewed by Andrey Borodin

Discussion: https://postgr.es/m/ec295792-a69f-350f-6287-25a20e8f31d5@gmail.com

Author: tglsfdc

contrib/pageinspect/heapfuncs.c

@@ -232,7 +232,7 @@ heap_page_items(PG_FUNCTION_ARGS)
 					int			bits_len;
 
 					bits_len =
-						((tuphdr->t_infomask2 & HEAP_NATTS_MASK) / 8 + 1) * 8;
+						BITMAPLEN(HeapTupleHeaderGetNatts(tuphdr)) * BITS_PER_BYTE;
 					values[11] = CStringGetTextDatum(
 									 bits_to_text(tuphdr->t_bits, bits_len));
 				}
@@ -434,24 +434,19 @@ tuple_data_split(PG_FUNCTION_ARGS)
 		int			bits_str_len;
 		int			bits_len;
 
-		bits_len = (t_infomask2 & HEAP_NATTS_MASK) / 8 + 1;
+		bits_len = BITMAPLEN(t_infomask2 & HEAP_NATTS_MASK) * BITS_PER_BYTE;
 		if (!t_bits_str)
 			ereport(ERROR,
 					(errcode(ERRCODE_DATA_CORRUPTED),
 					 errmsg("argument of t_bits is null, but it is expected to be null and %d character long",
-							bits_len * 8)));
+							bits_len)));
 
 		bits_str_len = strlen(t_bits_str);
-		if ((bits_str_len % 8) != 0)
-			ereport(ERROR,
-					(errcode(ERRCODE_DATA_CORRUPTED),
-					 errmsg("length of t_bits is not a multiple of eight")));
-
-		if (bits_len * 8 != bits_str_len)
+		if (bits_len != bits_str_len)
 			ereport(ERROR,
 					(errcode(ERRCODE_DATA_CORRUPTED),
 					 errmsg("unexpected length of t_bits %u, expected %d",
-							bits_str_len, bits_len * 8)));
+							bits_str_len, bits_len)));
 
 		/* do the conversion */
 		t_bits = text_to_bits(t_bits_str, bits_str_len);