libelf: If e_phnum is zero then set e_phoff also to zero.

libelf: If e_phnum is zero then set e_phoff also to zero.

If phnum is zero make sure e_phoff is also zero and not some random value.
That would cause trouble in update_file. This could happen when ELF_F_LAYOUT
is set and the user copied over a ehdr from a bogus ELF file where the phdrs
are unreadable. In that case trying to write out the new ELF image would
crash trying to follow the bogus e_phdr value.

Signed-off-by: Mark Wielaard <mjw@redhat.com>