8svx: fix crash

Fixes Ticket1377

Found-by: Piotr Bandurski <ami_stuff@o2.pl>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 03ce421c13)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/8svx.c

@@ -47,7 +47,7 @@ typedef struct EightSvxContext {
     /* buffer used to store the whole audio decoded/interleaved chunk,
      * which is sent with the first packet */
     uint8_t *samples;
-    size_t samples_size;
+    int64_t samples_size;
     int samples_idx;
 } EightSvxContext;