ext4: fix potential unnitialized variable

Smatch complains "err" can be uninitialized in the caller.

    fs/ext4/indirect.c:349 ext4_alloc_branch()
    error: uninitialized symbol 'err'.

Set the error to zero on the success path.

Fixes: 8016e29f

 ("ext4: fast commit recovery path")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/363a4673-0fb8-4adf-b4fb-90a499077276@moroto.mountain
Signed-off-by: Theodore Ts'o <tytso@mit.edu>