Fix szimek#480 - Fix race condition / edge case in _strokeUpdate wher…

…e this._data can sometimes be []

src/signature_pad.ts

@@ -265,6 +265,13 @@ export default class SignaturePad {
   }
 
   private _strokeUpdate(event: MouseEvent | Touch): void {
+    if (this._data.length === 0) {
+      // This can happen if clear() was called while a signature is still in progress,
+      // or if there is a race condition between start/update events.
+      this._strokeBegin(event)
+      return
+    }
+
     const x = event.clientX;
     const y = event.clientY;