x11grab: fix a memory leak exposed by valgrind

When using "-f x11grab -i :0.0" valgrind reports a definitely lost
memory block with this message:

==31544== 5 bytes in 1 blocks are definitely lost in loss record 1 of 2
==31544==    at 0x4026E68: memalign (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==31544==    by 0x4026F17: posix_memalign (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==31544==    by 0x60D399A: av_malloc (in /usr/lib/x86_64-linux-gnu/libavutil.so.51.22.1)
==31544==    by 0x60D3A70: av_strdup (in /usr/lib/x86_64-linux-gnu/libavutil.so.51.22.1)
==31544==    by 0x4A2BE58: ??? (in /usr/lib/x86_64-linux-gnu/libavdevice.so.53.2.0)
==31544==    by 0x506D29E: avformat_open_input (in /usr/lib/x86_64-linux-gnu/libavformat.so.53.21.0)
==31544==    by 0x400A80: main (in /home/ao2/WIP/am7xxx-play/tests/a.out)

The 5 bytes lost are the ones from param = av_strdup(":0.0"), so let's
free param in the exit path.

Also check the av_strdup() return value.

Note: calling av_free(param) even when av_strdup() fails and param is
NULL is OK and keeps the code simpler without adding another label to
skip av_free().

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

libavdevice/x11grab.c

@@ -169,6 +169,9 @@ x11grab_read_header(AVFormatContext *s1)
     AVRational framerate;
 
     param = av_strdup(s1->filename);
+    if (!param)
+        goto out;
+
     offset = strchr(param, '+');
     if (offset) {
         sscanf(offset, "%d,%d", &x_off, &y_off);
@@ -319,6 +322,7 @@ x11grab_read_header(AVFormatContext *s1)
     st->codec->bit_rate = x11grab->frame_size * 1/av_q2d(x11grab->time_base) * 8;
 
 out:
+    av_free(param);
     return ret;
 }