libavformat/ftp: Do not leak memory in routine ftp_features

Setting the pointer to NULL inside both ftp_send_command
and ftp_features is redundant. Generally always setting to
NULL in ftp_send_command seems safer, but throughout the file
that parameter was always passed initialized. So I do it here
too for consistency.

Should fix CID1231988 (RESOURCE_LEAK)

OKed-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
Signed-off-by: Alexander Strasser <eclipse7@gmx.net>