Wrong mappings when `update` or `overwrite` end with new line

[Simon H]

const s = new MagicString('foobar');
s.update(2, 3, 'od\n');
s.update(4, 5, 'a\nnd\n');

does produce wrong mappings. When the string passed to update or overwrite ends with a new line, the mappings will contain two entries for column zero. This happens because in #261 the loop does not bail if the last line is empty, filling two lines with the same mapping.
This is currently blocking Svelte language tools from bumping magic-string to the latest version.