ld crash when PHDRS uses undefined names

[Roland McGrath]

> Approved - please apply.

Done.

> Note - it would be cleaner if the phdr_types[] table had a direct
> mapping between the name and its integer value, so that you did not 
> have to have the special case code for the PT_GNU_EH_FRAME.  Still
> this is not a high priority change.

I agree.  But I was fixing a crash, not trying to improve the source,
so I just made the minimal necessary change.