fix insert position when erasing a code point in insert mode

src/core/Buffer.cc

@@ -196,5 +196,9 @@ void Buffer::erase_code_point(const Buffer::Iterator & position)
                 iterator->warp(-(ssize_t)bytes, lines);
             }
         }
+        if (m_insert_position > position.offset())
+        {
+            m_insert_position -= bytes;
+        }
     }
 }