ubifs bad superblock bug

This patch fixes an issue when ubifs reads a bad superblock. Later it
tries to free memory, that was not allocated, which freezes u-boot.
This is fixed by looking for a non null pointer before free.

The message I got before u-boot freezes:
UBI: max/mean erase counter: 53/32
UBIFS: mounted UBI device 0, volume 1, name "rootfs"
UBIFS: mounted read-only
UBIFS: file system size:   49140 bytes (50319360 KiB, 0 MiB, 49140 LEBs)
UBIFS: journal size:       49 bytes (6838272 KiB, 0 MiB, 6678 LEBs)
UBIFS: media format:       w4/r0 (latest is w4/r0)
UBIFS: default compressor: LZO
UBIFS: reserved for root:  0 bytes (0 KiB)
UBIFS error (pid 0): ubifs_read_node: bad node type (255 but expected 9)
UBIFS error (pid 0): ubifs_read_node: bad node at LEB 330:13104
UBIFS error (pid 0): ubifs_iget: failed to read inode 1, error -22
Error reading superblock on volume 'ubi:rootfs'!

Signed-off-by: Lars Poeschel <larsi@wh2.tu-dresden.de>
Cc: Kyungmin Park <kmpark@infradead.org>
Signed-off-by: Stefan Roese <sr@denx.de>