Fix non-deterministic archive type detection

Due to analysis of partly uninitialized read-ahead buffer
(short_read call), we sometimes mistakenly classified very small
compressed archives as non-compressed; which in turn caused
extraction failure.

* src/buffer.c (check_compressed_archive): Don't assume that
archives smaller than BLOCKSIZE could be non-compressed, as tar
header always has at least one block.

src/buffer.c

@@ -402,10 +402,12 @@ check_compressed_archive (bool *pshort)
   /* Restore global values */
   read_full_records = sfr;
 
-  if ((strcmp (record_start->header.magic, TMAGIC) == 0 ||
-       strcmp (record_start->buffer + offsetof (struct posix_header, magic),
-	       OLDGNU_MAGIC) == 0) &&
-      tar_checksum (record_start, true) == HEADER_SUCCESS)
+  if (record_start != record_end /* no files smaller than BLOCKSIZE */
+      && (strcmp (record_start->header.magic, TMAGIC) == 0
+          || strcmp (record_start->buffer + offsetof (struct posix_header,
+                                                      magic),
+                     OLDGNU_MAGIC) == 0)
+      && tar_checksum (record_start, true) == HEADER_SUCCESS)
     /* Probably a valid header */
     return ct_tar;