Detect and handle invalid number of FATs

If the number of FATs field in the boot sector is zero, give
an appropriate error code.

Obtained from: Android https://android.googlesource.com/platform/external/fsck_msdos/+/6c29bbe8d58e6fe8755935a04166ecf82ff31f47%5E%21/
MFC after: 2 weeks