powerpc/eeh: Check handle_eeh_events() return value

Function eeh_event_handler() dereferences the pointer returned by
handle_eeh_events() without checking, causing a crash if NULL was
returned, which is expected in some situations.

This patch fixes this bug by checking for the value returned by
handle_eeh_events() before dereferencing it.

Signed-off-by: Kleber Sacilotto de Souza <klebers@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: stable@vger.kernel.org [v3.4+]