fix : "ion-slides" - fast rerendering causes a crash

[mickadoua]

add condition, to check if slider exist before remove

Short description of what this resolves:

bug on fast rerendering #10830

Changes proposed in this pull request:

• check if slider exist before clean

Ionic Version: 2.2.1

Fixes: #10830

[mickadoua]

any update ?

[Creawebs]

Fixes: #10830
Is lacking on
function destroyLoop(s) {

eachChild(s._wrapper, '.' + CLS.slide + '.' + CLS.slideDuplicate, function (ele) {
    ele.parentElement.removeChild(ele);
});
for (var /** @type {?} */ i = 0; i < s._slides.length; i++) {
    s._slides[i].removeAttribute('data-swiper-slide-index');
}

}

It shows the same error when calling s._slides.length, same fix might apply.