rtc: ds1307: check for failed memory allocation on wdt

Currently a failed memory allocation will lead to a null pointer
dereference on point wdt.  Fix this by checking for a failed
allocation and just returning.

Addresses-Coverity: ("Dereference null return")
Fixes: fd90d48d

 ("rtc: ds1307: add support for watchdog timer on ds1388")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20200403110437.57420-1-colin.king@canonical.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>