MFC: r340240:
loader: ptable_open() check for ptable_cd9660read result is wrong

The ptable_*read() functions return NULL on read errors (and partition table
closed as an side effect). The ptable_open must check the return value and
act properly.

PR: 232483
Reported by: lev
Reviewed by: lev,cem
Differential Revision: https://reviews.freebsd.org/D17890