powerpc: Protect against NULL pointer deref in phyp-dump code

print_dump_header() will be called at least once with a NULL pointer in
a normal boot sequence.  If DEBUG is defined then we will dereference
the pointer and crash.  Add a quick fix to exit early in the NULL pointer
case.

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Acked-by: Manish Ahuja <mahujam@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>