libavformat/dashdec: Fix issue with dash on Windows

Use xmlFree instead of av_freep

snip from libxml2:

 * xmlGetProp:
...
 * Returns the attribute value or NULL if not found.
 *     It's up to the caller to free the memory with xmlFree().

According to libxml2, you are supposed to use xmlFree instead of free
on the pointer returned by it, and also using av_freep on Windows will
call _aligned_free instead of normal free, causing _aligned_free to raise
SIGTRAP and crashing ffmpeg and ffplay.

Signed-off-by: Christopher Degawa <ccom@randomderp.com>
(cherry picked from commit 0117d5aa03)
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/dashdec.c

@@ -1155,7 +1155,7 @@ static int parse_manifest_adaptationset(AVFormatContext *s, const char *url,
     }
 
 err:
-    av_freep(&c->adaptionset_lang);
+    xmlFree(c->adaptionset_lang);
     return ret;
 }