power: supply: cpcap-charger: Fix power_supply_put on null battery pointer

Currently if the pointer battery is null there is a null pointer
dereference on the call to power_supply_put.  Fix this by only
performing the put if battery is not null.

Addresses-Coverity: ("Dereference after null check")
Fixes: 4bff91bb ("power: supply: cpcap-charger: Fix missing power_supply_put()")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>