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: c48c9f7f ("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.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/exfat/exfat_cache.c

@@ -369,6 +369,8 @@ static s32 __FAT_write(struct super_block *sb, u32 loc, u32 content)
 				FAT_modify(sb, sec);
 
 				fat_sector = FAT_getblk(sb, ++sec);
+				if (!fat_sector)
+					return -1;
 				fat_sector[0] = (u8)((fat_sector[0] & 0xF0) |
 						     (content >> 8));
 			} else {