cbfs: Adjust return value of file_cbfs_next_file()

At present this uses a true return to indicate it found a file. Adjust it
to use 0 for this, so it is consistent with other functions.

Update its callers accordingly and add a check for malloc() failure in
file_cbfs_fill_cache().

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>