[fix] fix an arithmetic bug and update README files.

Author: winshining

README.CN.md

@@ -306,6 +306,6 @@ nginx-http-flv-module包含了[nginx-rtmp-module](https://github.com/arut/nginx-
         }
     }
 
-# RHEL 6（CentOS 6）和7（CentOS 7）上的rpm安装包
+# RHEL/CentOS 6和7上的RPM安装包
 
 详情见[nginx-http-flv-module-packages](https://github.com/winshining/nginx-http-flv-module-packages)。

README.md

@@ -307,6 +307,6 @@ The directive `worker_processes` of value 1 is preferable to other values, becau
         }
     }
 
-# rpm packages on RHEL 6 (CentOS 6) and 7 (CentOS 7)
+# RPM packages on RHEL/CentOS 6 and 7
 
 Please refer to [nginx-http-flv-module-packages](https://github.com/winshining/nginx-http-flv-module-packages).

ngx_rtmp_handler.c

@@ -722,10 +722,8 @@ ngx_rtmp_send_message(ngx_rtmp_session_t *s, ngx_chain_t *out,
         ngx_uint_t priority)
 {
     ngx_uint_t                      nmsg;
-    ssize_t                         delta;
 
-    delta = s->out_last - s->out_pos;
-    nmsg = (delta >= 0 ? delta : -delta) % s->out_queue + 1;
+    nmsg = (s->out_last + s->out_queue - s->out_pos) % s->out_queue + 1;
 
     if (priority > 3) {
         priority = 3;