ac3enc: fix 'warning: block0 may be used uninitialized in this function'

The pointer is also initialized to NULL for safety.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/ac3enc_template.c

@@ -336,7 +336,7 @@ static void compute_rematrixing_strategy(AC3EncodeContext *s)
 {
     int nb_coefs;
     int blk, bnd;
-    AC3Block *block, *block0;
+    AC3Block *block, *block0 = NULL;
 
     if (s->channel_mode != AC3_CHMODE_STEREO)
         return;