avformat/aadec: Replace strncpy by av_strlcpy

avformat/aadec: Replace strncpy by av_strlcpy

While this usage of strncpy is correct, said function nevertheless has
the disadvantage of not automatically ensuring that the destination
string is zero-terminated. So av_strlcpy should be preferred.
This also removes a -Wstringop-truncation warning from GCC (it doesn't
matter whether the buffer is truncated, as long as it can fit all
the names of the supported codecs).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>