mss3: prevent AC state from becoming invalid in rac_normalise()

Fixes division by zero

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

libavcodec/mss3.c

@@ -295,7 +295,7 @@ static void rac_normalise(RangeCoder *c)
             c->low |= *c->src++;
         } else if (!c->low) {
             c->got_error = 1;
-            return;
+            c->low = 1;
         }
         if (c->range >= RAC_BOTTOM)
             return;