fs: ext4: fix crash on ext4ls

fs: ext4: fix crash on ext4ls

Found a crash while issuing ext4ls with a non-existent directory.
Crash test:

=> ext4ls mmc 0 1
** Can not find directory. **
data abort
pc : [<3fd7c2ec>]          lr : [<3fd93ed8>]
reloc pc : [<26f142ec>]    lr : [<26f2bed8>]
sp : 3f963338  ip : 3fdc3dc4     fp : 3fd6b370
r10: 00000004  r9 : 3f967ec0     r8 : 3f96db68
r7 : 3fdc99b4  r6 : 00000000     r5 : 3f96dc88  r4 : 3fdcbc8c
r3 : fffffffa  r2 : 00000000     r1 : 3f96e0bc  r0 : 00000002
Flags: nZCv  IRQs off  FIQs off  Mode SVC_32
Resetting CPU ...

resetting ...

Tested on SAMA5D2_Xplained board (sama5d2_xplained_mmc_defconfig)

Looks like crash is introduced by commit:
"fa9ca8a" fs/ext4/ext4fs.c: Free dirnode in error path of ext4fs_ls

Issue is that dirnode is not initialized, and then freed if the call
to ext4_ls fails. ext4_ls will not change the value of dirnode in this case
thus we have a crash with data abort.

I added initialization and a check for dirname being NULL.

Fixes: "fa9ca8a" fs/ext4/ext4fs.c: Free dirnode in error path of ext4fs_ls
Cc: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Cc: Tom Rini <trini@konsulko.com>
Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Reviewed-by: Tom Rini <trini@konsulko.com>