oomd: Prevent corruption of cgroup paths in Killed signal

oomd: Prevent corruption of cgroup paths in Killed signal

While looking at oomd behaviour in production I noticed that I always
get garbage cgroup paths for the Killed event. Looking more closely, I
noticed that while the signature is (string cgroup, string reason), we
currently erroneously pass the `OomdCGroupContext*` pointer itself as
the first argument to sd_bus_emit_signal(), rather than the ctx->path
string it contains.

The in-memory layout on affected machines in my case is:

    struct OomdCGroupContext {
        unsigned n_ref;
        /* padding */
        char *path;
        /* ... */
    }

...which explains the control characters, since they're garbage from
parsing n_ref, the path pointer, and later fields. At runtime, sd-bus
treats ctx as `const char *` and reads struct bytes as string data,
resulting in garbage being sent.

Pass ctx->path correctly so listeners receive the valid cgroup path.