[server] fix Clang compiler warning to remove extraneous var increment

utils.c:217:13: warning: variable 'i' is incremented both in the loop header and in the loop
      body [-Wfor-loop-analysis]
            i++;
            ^
utils.c:213:34: note: incremented here
        for (i=0; i<strlen(str); i++)

server/utils.c

@@ -214,7 +214,6 @@ is_digits(const char * const str)
         {
             if(!isdigit((int)(unsigned char)str[i]))
                 return 0;
-            i++;
         }
     }
     return 1;