libavformat/dashdec: Fix issue with dash on Windows

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>