ac_str: Prevent potential segfault in ac_str_freev()
ac_str: Prevent potential segfault in ac_str_freev()

Bail out if the passed in stringv is NULL.

Signed-off-by: Andrew Clayton <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@digital-domain.net>

ac000/libac