avcodec/rv34: assert that size is not 0 in rv34_gen_vlc_ext()

Helps: CID1548380 Uninitialized scalar variable

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit e5098589b0)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/rv34.c

@@ -97,6 +97,8 @@ static void rv34_gen_vlc(const uint8_t *bits, int size, VLC *vlc, const uint8_t
     uint16_t cw[MAX_VLC_SIZE];
     int maxbits;
 
+    av_assert1(size > 0);
+
     for (int i = 0; i < size; i++)
         counts[bits[i]]++;