qemu-io: Don't leak pattern file in error path

qemu_io_alloc_from_file() needs to close the pattern file even if some
error occurred.

Setting f = NULL in the success path and checking it for NULL in the
error path isn't strictly necessary at this point, but let's do it
anyway in case someone later adds a 'goto error' after closing the file.

Coverity: CID 1405303
Fixes: 4d731510
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>