tighten buffer bounds in imgact_binmisc_populate_interp

We must ensure there's space for the terminating null in the temporary
buffer in imgact_binmisc_populate_interp().

Note that there's no buffer overflow here because xbe->xbe_interpreter's
length and null termination is checked in imgact_binmisc_add_entry()
before imgact_binmisc_populate_interp() is called. However, the latter
should correctly enforce its own bounds.

Reviewed by: sbruno
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D10042