Skip data concatenation for z instruction in collapseRepeated

Example path part:

```
-32.5 -82zt0 0zM371
```

It will be stripped to:

```
-32.5 -82zzM371
```

So it will be crashed (with no output) in convertPathData.js::collapseRepeated:

```js
prev.data = prev.data.concat(item.data);
```

Full example to reproduce:

```xml
<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd" >
<svg xmlns="http://www.w3.org/2000/svg">
<path d="M188 -12q-26 1 -52.5 7q-26.5 6 -49.5 14.5q-23 8.5 -41.5 17.5q-18.5 9 -27.5 17q8 33 19.5 63q11.5 30 23.5 50l26 0l40 -97q19 -19 73 -20l61 281q-26 17 -5    1 34.5q-25 17.5 -44.5 40q-19.5 22.5 -32 51q-12.5 28.5 -12.5 67.5q0 36 14 70.5q14 34.5 41.5 61.5q27.5 27 69 44.5q41.5 17.5 96.5 20.5l11 49h52l-11 -49q49 -2 84 -11q    35 -9 60 -21q-9 -33 -25 -55.5q-16 -22.5 -31 -37.5l-26 0l-36 68q-8 1 -17 2q-9 1 -19 3l-54 -257q24 -16 48.5 -33.5q24.5 -17.5 44 -41.5q19.5 -24 31.5 -56q12 -32 12 -7    5q0 -39 -15.5 -74q-15.5 -35 -44.5 -62.5q-29 -27.5 -71 -46q-42 -18.5 -94 -23.5l-12 -60h-52zM213 543q0 -41 20 -66.5q20 -25.5 50 -45.5l49 228q-54 -4 -86.5 -34q-32.5     -30 -32.5 -82zt0 0zM371 167q0 42 -19.5 71q-19.5 29 -48.5 52l-53 -247q24 4 45.5 14q21.5 10 38.5 25.5q17 15.5 27 36.5t10 48z" />
</svg>
```