ext4: fix potential null pointer dereference in ext4_free_inode

Fix potential null pointer dereferencing problem caused by e43bb4e612
("ext4: decrement free clusters/inodes counters when block group declared bad")

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Ashish Sangwan <a.sangwan@samsung.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Lukas Czerner <lczerner@redhat.com>

fs/ext4/ialloc.c

@@ -338,7 +338,7 @@ out:
 			fatal = err;
 	} else {
 		ext4_error(sb, "bit already cleared for inode %lu", ino);
-		if (!EXT4_MB_GRP_IBITMAP_CORRUPT(grp)) {
+		if (gdp && !EXT4_MB_GRP_IBITMAP_CORRUPT(grp)) {
 			int count;
 			count = ext4_free_inodes_count(sb, gdp);
 			percpu_counter_sub(&sbi->s_freeinodes_counter,