libebl: Check NT_PLATFORM core notes contain a zero terminated string.

Most strings in core notes are fixed size. But NT_PLATFORM contains just
a variable length string. Check that it is actually zero terminated
before passing to readelf to print.

https://sourceware.org/bugzilla/show_bug.cgi?id=24089



Signed-off-by: Mark Wielaard <mark@klomp.org>