rtmp: tried to replicate C behaviour by returning would be no of bytes written

rtmp/rtmp.go

@@ -2189,7 +2189,7 @@ func C_RTMP_Write(r *C_RTMP, buf []byte) int {
 				return -1
 			}
 			if len(buf) < 4 {
-				break
+				return size + (len(buf) - 4)
 			}
 			buf = buf[4:]
 		}