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>

arch/powerpc/kernel/hw_breakpoint.c

@@ -111,7 +111,7 @@ void arch_unregister_hw_breakpoint(struct perf_event *bp)
 	 * and the single_step_dabr_instruction(), then cleanup the breakpoint
 	 * restoration variables to prevent dangling pointers.
 	 */
-	if (bp->ctx->task)
+	if (bp->ctx && bp->ctx->task)
 		bp->ctx->task->thread.last_hit_ubp = NULL;
 }