fix: divisor cannot be 0 (#72)

* Divisor cannot be 0

* fix another

* some test case

* update

Author: yoyo837

src/ScrollBar.tsx

@@ -126,7 +126,7 @@ export default class ScrollBar extends React.Component<ScrollBarProps, ScrollBar
       const enableScrollRange = this.getEnableScrollRange();
       const enableHeightRange = this.getEnableHeightRange();
 
-      const ptg = newTop / enableHeightRange;
+      const ptg = enableHeightRange ? newTop / enableHeightRange : 0;
       const newScrollTop = Math.ceil(ptg * enableScrollRange);
       this.moveRaf = raf(() => {
         onScroll(newScrollTop);
@@ -153,19 +153,22 @@ export default class ScrollBar extends React.Component<ScrollBarProps, ScrollBar
 
   getEnableScrollRange = () => {
     const { scrollHeight, height } = this.props;
-    return scrollHeight - height;
+    return scrollHeight - height || 0;
   };
 
   getEnableHeightRange = () => {
     const { height } = this.props;
     const spinHeight = this.getSpinHeight();
-    return height - spinHeight;
+    return height - spinHeight || 0;
   };
 
   getTop = () => {
     const { scrollTop } = this.props;
     const enableScrollRange = this.getEnableScrollRange();
     const enableHeightRange = this.getEnableHeightRange();
+    if (scrollTop === 0 || enableScrollRange === 0) {
+      return 0;
+    }
     const ptg = scrollTop / enableScrollRange;
     return ptg * enableHeightRange;
   };