commands/file: Fix NULL dereference in the knetbsd tests

commands/file: Fix NULL dereference in the knetbsd tests

The pointer returned by grub_elf_file() is not checked to verify it is
not NULL before use. A NULL pointer may be returned when the given file
does not have a valid ELF header.

Fixes: https://savannah.gnu.org/bugs/?61960
Signed-off-by: Glenn Washburn <development@efficientek.com>
Signed-off-by: Lukas Fink <lukas.fink1@gmail.com>
Reviewed-by: Ross Philipson <ross.philipson@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>