a64multienc: change mc_frame_counter to unsigned

GCC cannot proof that c->mc_frame_counter is always >- 0, changing the
type from int to unsigned fixes following warning:

libavcodec/a64multienc.c: In function ‘a64multi_encode_frame’:
libavcodec/a64multienc.c:342:17: warning: ‘buf’ may be used uninitialized in this function[-Wuninitialized]

Signed-off-by: Janne Grunau <janne-libav@jannau.net>
