widget: ensure buffer.Read does nothing to read zero bytes

This commit ensures that the edit buffer used by widget.Editor
does not get EOF when trying to read zero bytes from the
underlying buffer, which eliminates a panic when calling
Editor.SelectedText().

Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

widget/editor_test.go

@@ -121,6 +121,9 @@ func assertCaret(t *testing.T, e *Editor, line, col, bytes int) {
 	if bytes != caretBytes {
 		t.Errorf("caret at buffer position %d, expected %d", caretBytes, bytes)
 	}
+	// Ensure that SelectedText() does not panic no matter what the
+	// editor's state is.
+	_ = e.SelectedText()
 }
 
 type editMutation int