avutil/aes: Don't use out-of-bounds index

Up until now, av_aes_init() uses a->round_key[0].u8 + t
as dst of memcpy where it is intended for t to greater
than 16 (u8 is an uint8_t[16]); given that round_key itself
is an array, it is actually intended for the dst to be
in a latter round_key member. To do this properly,
just cast a->round_key to unsigned char*.

This fixes the srtp, aes, aes_ctr, mov-3elist-encrypted,
mov-frag-encrypted and mov-tenc-only-encrypted
FATE-tests with (Clang-)UBSan.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavutil/aes.c

@@ -253,7 +253,7 @@ int av_aes_init(AVAES *a, const uint8_t *key, int key_bits, int decrypt)
                     tk[j][i] ^= sbox[tk[j - 1][i]];
         }
 
-        memcpy(a->round_key[0].u8 + t, tk, KC * 4);
+        memcpy((unsigned char*)a->round_key + t, tk, KC * 4);
     }
 
     if (decrypt) {