mmsh/mmsh_close: use av_freep()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
mmsh/mmsh_close: use ffurl_closep()

avoid using freed pointers

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
mmsh: dont close context on seeking failure

Fixes Ticket2581

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