Merge pull request #2692 from taosdata/hotfix/TD-936

coverity issue

src/util/src/tutil.c

@@ -239,7 +239,10 @@ char* strntolower(char *dst, const char *src, int32_t n) {
   char quote = 0, *p = dst, c;
 
   assert(dst != NULL);
-
+  if (n == 0) {
+    *p = 0;
+    return dst;
+  } 
   for (c = *src++; n-- > 0; c = *src++) {
     if (esc) {
       esc = 0;