mov: fix global unicode convertion array overflow.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mov.c

@@ -160,7 +160,7 @@ static int mov_read_mac_string(MOVContext *c, AVIOContext *pb, int len,
         uint8_t t, c = avio_r8(pb);
         if (c < 0x80 && p < end)
             *p++ = c;
-        else
+        else if (p < end)
             PUT_UTF8(mac_to_unicode[c-0x80], t, if (p < end) *p++ = t;);
     }
     *p = 0;