powerpc/hw_breakpoints: Fix incorrect pointer access

If arch_validate_hwbkpt_settings() fails, bp->ctx won't be valid and the
kernel panics. Add a check to fix this.

Reported-by: Edjunior Barbosa Machado <emachado@linux.vnet.ibm.com>
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>