fstyp: Fix some memory safety bugs

In the hammer2 label reader, make sure to check for a NULL return from
read_buf().

In the NTFS label reader,

• Avoid an infinite loop if a record length is 0.
• Avoid walking past the end of the buffer.
• When a label is found, avoid reading past the end of the buffer.

PR: 278281
Reviewed by: emaste
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D47292