added bounds check to prevent panic

[v4nderstruck]

addresses my own issue here #4038

simple bounds check should do the trick

[the-mikedavis]

This prevents the panic but doesn't solve the underlying issue that causes the panic.

We panic here because the offset points to a part of the text that doesn't exist after :formating. Instead of handling the panic here, we should add a call to view.ensure_cursor_in_view(doc, editor.config().scrolloff) to the :format implementation (in this block:

helix/helix-term/src/commands.rs

Lines 2510 to 2515 in 8a7a6e4

	doc.apply(&format, view_id);
	doc.append_changes_to_history(view_id);
	doc.detect_indent_and_line_ending();
	if let Modified::SetUnmodified = modified {
	doc.reset_modified();
	}

)