PG-1551 Handle errors and short reads while reading WAL

Do not try to decrypt data not actually read by returning directly if we
reached the end of file or got an error. If we get a short read we also
return directly which is safe snice pg_tde_crypt() supports any offset,
not just multiples of the AES block size. This likely makes it a bit
trickier to move to OpenSSL's built-in CTR which we probably want to do
in the future but let's cross that bridge when we get to it.

contrib/pg_tde/src/access/pg_tde_xlog_smgr.c

@@ -270,11 +270,11 @@ tdeheap_xlog_seg_read(int fd, void *buf, size_t count, off_t offset,
 		 count, offset, offset, LSN_FORMAT_ARGS(segno));
 #endif
 
-	/*
-	 * Read data from disk
-	 */
 	readsz = pg_pread(fd, buf, count, offset);
 
+	if (readsz <= 0)
+		return readsz;
+
 	if (!keys)
 	{
 		/* cache is empty, try to read keys from disk */
@@ -302,7 +302,7 @@ tdeheap_xlog_seg_read(int fd, void *buf, size_t count, off_t offset,
 #endif
 
 	XLogSegNoOffsetToRecPtr(segno, offset, segSize, data_start);
-	XLogSegNoOffsetToRecPtr(segno, offset + count, segSize, data_end);
+	XLogSegNoOffsetToRecPtr(segno, offset + readsz, segSize, data_end);
 
 	/*
 	 * TODO: this is higly ineffective. We should get rid of linked list and
@@ -339,7 +339,7 @@ tdeheap_xlog_seg_read(int fd, void *buf, size_t count, off_t offset,
 				/* We have reached the end of the segment */
 				if (dec_end == 0)
 				{
-					dec_end = offset + count;
+					dec_end = offset + readsz;
 				}
 
 				dec_sz = dec_end - dec_off;