zram: correctly handle all next_arg() cases

When supplied buffer does not have assignment sign next_arg() sets `val`
pointer to NULL, so we cannot dereference it.  Add a NULL pointer test to
handle `param` case, in addition to `*val` test, which handles cases when
param has no value assigned to it: `param=`.

Link: https://lkml.kernel.org/r/20230103030119.1496358-1-senozhatsky@chromium.org


Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>