s4:lib:messaging:handlers fix cast-align warning

s4:lib:messaging:handlers fix cast-align warning

Have added asserts to check the alignment, as do_inject_build and do_sleep are
only enabled in developer and selftest builds.

The pointers sig_p  and seconds_p where added for code readability, you can
dereference the result of discard_align_p directly i.e.
	int sig = *discard_align_p(int, data->data);
but that's not immediately obvious

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Jennifer Sutton <jennifersutton@catalyst.net.nz>