qemu-img: free memory before re-assign

qemu-img: free memory before re-assign

collect_image_check() is called twice in img_check(), the filename/format will be alloced without free the original memory.
It is not a big deal since the process will exit anyway, but seems like a clean code and it will remove the warning spotted by asan.

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Pan Nengyuan <pannengyuan@huawei.com>
Message-Id: <20200227012950.12256-3-pannengyuan@huawei.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>