Fix debug-panic in ScrollArea if contents fit without scrolling (#7440)

If the ScrollArea's contents are smaller than the inner rect, but the
scrollbar is set to always visible, clicking on it led to a remap from
an empty range to calculate the new offset, which triggered a debug
assertion in the remap function, because the result is indeterminate.

Since in this case there is no need to scroll, we just skip the remap
and set the offset to 0 directly.

* Closes <https://github.com/emilk/egui/issues/7362>
* [x] I have followed the instructions in the PR template

crates/egui/src/containers/scroll_area.rs

@@ -1315,11 +1315,13 @@ impl Prepared {
                     });
 
                 let new_handle_top = pointer_pos[d] - *scroll_start_offset_from_top_left;
-                state.offset[d] = remap(
-                    new_handle_top,
-                    scroll_bar_rect.min[d]..=(scroll_bar_rect.max[d] - handle_rect.size()[d]),
-                    0.0..=max_offset[d],
-                );
+                let handle_travel =
+                    scroll_bar_rect.min[d]..=(scroll_bar_rect.max[d] - handle_rect.size()[d]);
+                state.offset[d] = if handle_travel.start() == handle_travel.end() {
+                    0.0
+                } else {
+                    remap(new_handle_top, handle_travel, 0.0..=max_offset[d])
+                };
 
                 // some manual action taken, scroll not stuck
                 state.scroll_stuck_to_end[d] = false;