[IA64] Take defensive stance on ia64_pal_get_brand_info()

Stephane thought he saw a problem here (but was just confused
by the return value from ia64_pal_get_brand_info()).  But we
should be more defensive here in case an prototype PAL for
a future processor doesn't implement this PAL call.

Signed-off-by: Tony Luck <tony.luck@intel.com>