uid-range: add asserts to document overflow safety in coalesce

Coverity flags the x->start + x->nr and y->start + y->nr additions
as potential overflows. These are safe because uid_range_add_internal()
validates start + nr <= UINT32_MAX before inserting entries. Add asserts
to document this invariant for static analyzers.

CID#1548015

Follow-up for 8530dc4467

src/basic/uid-range.c

@@ -63,6 +63,10 @@ static void uid_range_coalesce(UIDRange *range) {
                                 break;
 
                         begin = MIN(x->start, y->start);
+
+                        /* Silence static analyzers, overflow is prevented by uid_range_add_internal() */
+                        assert(x->start <= UINT32_MAX - x->nr);
+                        assert(y->start <= UINT32_MAX - y->nr);
                         end = MAX(x->start + x->nr, y->start + y->nr);
 
                         x->start = begin;