Do more sanity-checking in unwind-dw2-fde.c

[Douglas B Rupp]

> > If the .eh_frame section is empty...
>
> Why would it be empty?
>
> /* Terminate the frame unwind info section with a 4byte 0 as a sentinel;
>    this would be the 'length' field in a real FDE.  */
> STATIC EH_FRAME_SECTION_CONST int __FRAME_END__[]
>      __attribute__ ((unused, mode(SI), section(EH_FRAME_SECTION_NAME),
>                      aligned(4)))
>      = { 0 };
>

VMS has its own crtbegin code (see config/alpha/vms-*crt*.c) and doesn't use
crtstuff.c.