multi_buffer: Assert char boundary for panic due to point_to_buffer_offset (#40777)

In an attempt to figure out what's wrong with `point_to_buffer_offset`
for crash https://github.com/zed-industries/zed/issues/40453. We want to
know which branch among these two is the bad one.

Release Notes:

- N/A

Co-authored-by: Lukas Wirth <lukas@zed.dev>

crates/multi_buffer/src/multi_buffer.rs

@@ -4454,10 +4454,23 @@ impl MultiBufferSnapshot {
             && region.has_trailing_newline
             && !region.is_main_buffer
         {
-            return Some((&cursor.excerpt()?.buffer, cursor.main_buffer_position()?));
+            let main_buffer_position = cursor.main_buffer_position()?;
+            let buffer_snapshot = &cursor.excerpt()?.buffer;
+            // remove this assert once we figure out the cause of the panics for #40453
+            buffer_snapshot
+                .text
+                .as_rope()
+                .assert_char_boundary(main_buffer_position);
+            return Some((buffer_snapshot, main_buffer_position));
         } else if buffer_offset > region.buffer.len() {
             return None;
         }
+        // remove this assert once we figure out the cause of the panics for #40453
+        region
+            .buffer
+            .text
+            .as_rope()
+            .assert_char_boundary(buffer_offset);
         Some((region.buffer, buffer_offset))
     }