cpukit/libblock: Close all valid FDs

This is intended to close any valid FDs that were opened, but 0 is also
a valid FD. This change ensures that a FD of 0 is also closed properly.

Coverity CID 1467408 (original)

Coverity CID 1616151 (followup)

off_by_one: Testing whether handle fd is strictly greater than zero is
suspicious. fd leaks when it is zero.

cpukit/libblock/src/bdpart-register.c

@@ -213,7 +213,7 @@ cleanup:
    * but fd may be -1. Coverity flagged passing a bad value to close().
    */
   free( logical_disk_name);
-  if (fd > 0) {
+  if (fd >= 0) {
     close( fd);
   }