Fix potential StringIndexOutOfBoundsException, fixes #1020 (#1023)

At present it's still unclear under which condition this issue was
triggered. It should no longer crash though, the UI might just behave a
bit strangely instead.

opentasks/src/main/java/org/dmfs/tasks/widget/DescriptionFieldView.java

@@ -332,9 +332,10 @@ private void bindItemView(final View itemView, final DescriptionItem item)
                     return true;
                 }
                 // split current
-                int sel = text.getSelectionStart();
-                String newText = text.getText().toString().substring(sel);
-                item.text = text.getText().toString().substring(0, sel);
+                String current = text.getText().toString();
+                int sel = Math.max(0, Math.min(current.length(), text.getSelectionStart()));
+                String newText = current.substring(sel);
+                item.text = current.substring(0, sel);
                 text.setText(item.text);
                 text.clearFocus();
                 // create new item with new test