tighten buffer bounds in imgact_binmisc_populate_interp

[emaste]

Summary

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

Note that there is not a possible 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, but the latter should correctly enforce its own bounds.