demux: fix libasf memory leak in extended stream properties

It seems that p_esp->p_sp allocated in https://code.videolan.org/videolan/vlc/-/blob/master/modules/demux/asf/libasf.c?ref_type=heads#L1014 is never freed.

This addes an appropriate call to FREENULL on this pointer in ASF_FreeObject_extended_stream_properties.

Disclaimer: We found this leak by fuzzing. A fuzzed file with this leak is leaky_file.asf