staging: exfat: check for null return from call to FAT_getblk

staging: exfat: check for null return from call to FAT_getblk

A call to FAT_getblk is missing a null return check which can
lead to a null pointer dereference.  Fix this by adding a null
check to match all the other FAT_getblk return sanity checks.

Addresses-Coverity: ("Dereference null return")
Fixes: c48c9f7ff32b ("staging: exfat: add exfat filesystem code to staging")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20190830175050.12706-1-colin.king@canonical.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>