rtmp: check that len(buf) is not less than 4 before buf = buf[4:]

rtmp/rtmp.go

@@ -2188,6 +2188,9 @@ func C_RTMP_Write(r *C_RTMP, buf []byte) int {
 			if ret == 0 {
 				return -1
 			}
+			if len(buf) < 4 {
+				break
+			}
 			buf = buf[4:]
 		}
 	}