scan-build thinks there could be a `NULL` pointer dereference of
`t->data` here. It’s wrong, so add an assertion to try and help it
understand the control flow.

The loop is exited as soon as a target is found whose weight is greater
than or equal to a random value between 0 and the sum of all the weights
in the set of remaining targets in the loop. By definition, the last
target in the loop always satisfies this condition, so a target will
always be chosen, and hence `t` will never be `NULL` within the loop.

`t->data` will never be `NULL` by construction of the target list.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #1767