Hotfix/sangshuduo/td 4892 taosdemo sub fetch (#6634)

* [TD-4892]<fix>: taosdemo subscribe fetch result.

* fix stbname length.

* restrict prefix length.

* submit empty

* fix minor code.

* fix crash if no result file.

src/kit/taosdemo/taosdemo.c

@@ -1204,23 +1204,24 @@ static void fetchResult(TAOS_RES *res, threadInfo* pThreadInfo) {
     return ;
   }
 
-  int   totalLen = 0;
-  char  temp[16000];
+  int64_t   totalLen = 0;
 
   // fetch the records row by row
   while((row = taos_fetch_row(res))) {
-    if ((strlen(pThreadInfo->filePath) > 0)
-            && (totalLen >= 100*1024*1024 - 32000)) {
-        appendResultBufToFile(databuf, pThreadInfo);
+    if (totalLen >= 100*1024*1024 - 32000) {
+        if (strlen(pThreadInfo->filePath) > 0)
+            appendResultBufToFile(databuf, pThreadInfo);
         totalLen = 0;
         memset(databuf, 0, 100*1024*1024);
     }
     num_rows++;
+    char  temp[16000] = {0};
     int len = taos_print_row(temp, row, fields, num_fields);
     len += sprintf(temp + len, "\n");
     //printf("query result:%s\n", temp);
     memcpy(databuf + totalLen, temp, len);
     totalLen += len;
+    debugPrint("totalLen: %"PRId64"\n", totalLen);
   }
 
   verbosePrint("%s() LN%d, databuf=%s resultFile=%s\n",