Add av_freep() to free new_elem to avoid memory leak.
Fixes: 3ca1e31e63 ("fftools/cmdutils: Atomically add elements to list of pointers, fix crash")
Signed-off-by: Jiasheng Jiang <jiashengjiangcool@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>