ffserver: Fixed ffserver to support large ffm files

ffm_read_write_index returns a 64bit value,

Github: Closes #185
  
An error occurred
 

ffserver.c

@@ -2622,6 +2622,7 @@ static int http_start_receive_data(HTTPContext *c)
 {
     int fd;
     int ret;
+    int64_t ret64;
 
     if (c->stream->feed_opened) {
         http_log("Stream feed '%s' was not opened\n",
@@ -2657,13 +2658,13 @@ static int http_start_receive_data(HTTPContext *c)
             return ret;
         }
     } else {
-        ret = ffm_read_write_index(fd);
+        ret64 = ffm_read_write_index(fd);
-        if (ret < 0) {
+        if (ret64 < 0) {
             http_log("Error reading write index from feed file '%s': %s\n",
                      c->stream->feed_filename, strerror(errno));
-            return ret;
+            return ret64;
         }
-        c->stream->feed_write_index = ret;
+        c->stream->feed_write_index = ret64;
     }
 
     c->stream->feed_write_index = FFMAX(ffm_read_write_index(fd),