优化签名组件，避免直接提交

one_stop_public/Signature/index.jsx

@@ -14,8 +14,13 @@ function dataURLtoFile (dataurl, filename) {
     var arr = dataurl.split(','),
         mime = arr[0].match(/:(.*?);/)[1],
         bstr = atob(arr[1]),
-        n = bstr.length,
-        u8arr = new Uint8Array(n);
+        n = bstr.length
+        
+        if(n<=2220){
+            message.error('请进行签名');
+            return null;
+        }
+        var u8arr = new Uint8Array(n);
     while (n--) {
         u8arr[n] = bstr.charCodeAt(n);
     }
@@ -61,7 +66,12 @@ export default class index extends Component {
     }
     trim=()=>{
         const formData = new FormData()
-        formData.append('file',dataURLtoFile(this.sigCanvas.toDataURL('image/png'),"sign.png"))
+        const file = dataURLtoFile(this.sigCanvas.toDataURL('image/png'))
+        if(file==null){
+           
+            return
+        }
+        formData.append('file',file,"sign.png")
         reqwest({
             url: config.uploadUrl,
             method: 'post',