x11grab: fix compilation

Found-by: ubitux
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavdevice/x11grab.c

@@ -323,7 +323,7 @@ x11grab_read_header(AVFormatContext *s1)
     st->codec->bit_rate = x11grab->frame_size * 1/av_q2d(x11grab->time_base) * 8;
 
 out:
-    av_free(param);
+    av_free(dpyname);
     return ret;
 }