lavf/rtmpproto: fix the playpath truncation if the len > 512

fix the playpath truncation if the len > 512
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Found-by: liuwenhuang <liuwenhuang@tencent.com>
Signed-off-by: Jun Zhao <barryjzhao@tencent.com>

libavformat/rtmpproto.c

@@ -48,7 +48,6 @@
 #endif
 
 #define APP_MAX_LENGTH 1024
-#define PLAYPATH_MAX_LENGTH 512
 #define TCURL_MAX_LENGTH 1024
 #define FLASHVER_MAX_LENGTH 64
 #define RTMP_PKTDATA_DEFAULT_SIZE 4096
@@ -2746,7 +2745,10 @@ reconnect:
     }
 
     if (!rt->playpath) {
-        rt->playpath = av_malloc(PLAYPATH_MAX_LENGTH);
+        int max_len = 1;
+        if (fname)
+            max_len = strlen(fname) + 5; // add prefix "mp4:"
+        rt->playpath = av_malloc(max_len);
         if (!rt->playpath) {
             ret = AVERROR(ENOMEM);
             goto fail;
@@ -2763,7 +2765,7 @@ reconnect:
                     fname[len - 4] = '\0';
                 rt->playpath[0] = 0;
             }
-            av_strlcat(rt->playpath, fname, PLAYPATH_MAX_LENGTH);
+            av_strlcat(rt->playpath, fname, max_len);
         } else {
             rt->playpath[0] = '\0';
         }