rkcommon.c: Drop pointless assignment

Assigning a variable to itself is not necessary. Drop this and also add a
check for malloc() failure.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Coverity (CID: 161418)
Fixes: 111bcc4f (rockchip: mkimage: pad the header to 8-bytes (using a 'nop') for RK3399)