scan-build was complaining that the `wc_buffer[old_n_wc]` in `cc =
COMBINING_CLASS (wc_buffer[old_n_wc])` could dereference memory off the
end of the initialised `wc_buffer` array. It came to this conclusion by
assuming that the result of `find_decomposition()` for one of the
`gunichar`s was a non-`NULL` empty string, so that iteration of the
decomposition loop didn’t append anything to `wc_buffer`.

I don’t think it’s possible for an iteration of the loop to *not* append
anything to `wc_buffer`. Unicode characters don’t decompose to nothing.

Indeed, the current code coverage for GLib says that the `if (n_wc > 0)`
branch is always taken, and at that point in the control flow, `n_wc <=
0` is never true.

So, add an assertion to check that progress is made (i.e. `n_wc` is
incremented by at least 1), and remove the unnecessary condition.

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

Helps: #1767