Do not attempt to decode LP_UNUSED line pointers

We previously called FormatItem on each item even when it was LP_UNUSED.
On x86_64, that was not a problem since the items seen in the test suite
had lp_len == 0, but on big-endian s390x, there are items with a
non-zero length and the decoder would complain about garbage.

"Good" case:

 Item   1 -- Length:    0  Offset:  773 (0x0305)  Flags: UNUSED
 Item   2 -- Length:    0  Offset:    0 (0x0000)  Flags: UNUSED

"Bad" case:

 Item   1 -- Length:    0  Offset:    0 (0x0000)  Flags: UNUSED
 Item   2 -- Length:  758  Offset:    0 (0x0000)  Flags: UNUSED
  Block Id: 0  linp Index: 402  Size: 7936
  Has Nulls: 1  Has Varwidths: 1

  Error: Index item size difference. Given <758>, Internal <7936>.

Author: df7cb

pg_filedump.c

@@ -1531,7 +1531,7 @@ FormatItemBlock(char *buffer,
 						   indent, indent, blockSize, bytesToFormat, itemOffset + itemSize);
 				exitCode = 1;
 			}
-			else
+			else if (itemFlags != LP_UNUSED)
 			{
 				HeapTupleHeader tuple_header;
 				TransactionId xmax;
@@ -1584,10 +1584,10 @@ FormatItemBlock(char *buffer,
 					/* Decode tuple data */
 					FormatDecode(&buffer[itemOffset], itemSize);
 				}
-
-				if (!isToast && x == maxOffset)
-					printf("\n");
 			}
+
+			if (!isToast && x == maxOffset)
+				printf("\n");
 		}
 	}
 }
@@ -1631,7 +1631,7 @@ FormatItem(char *buffer, unsigned int numBytes, unsigned int startIndex,
 
 			if (numBytes != IndexTupleSize(itup))
 			{
-				printf("  Error: Item size difference. Given <%u>, "
+				printf("  Error: Index item size difference. Given <%u>, "
 					   "Internal <%d>.\n", numBytes, (int) IndexTupleSize(itup));
 				exitCode = 1;
 			}
@@ -1660,7 +1660,7 @@ FormatItem(char *buffer, unsigned int numBytes, unsigned int startIndex,
 
 			if (numBytes != itup->size)
 			{
-				printf("  Error: Item size difference. Given <%u>, "
+				printf("  Error: SPGiST inner item size difference. Given <%u>, "
 					   "Internal <%d>.\n", numBytes, (int) itup->size);
 				exitCode = 1;
 			}
@@ -1733,7 +1733,7 @@ FormatItem(char *buffer, unsigned int numBytes, unsigned int startIndex,
 
 			if (numBytes != itup->size)
 			{
-				printf("  Error: Item size difference. Given <%u>, "
+				printf("  Error: SPGiST leaf item size difference. Given <%u>, "
 					   "Internal <%d>.\n", numBytes, (int) itup->size);
 				exitCode = 1;
 			}