avcodec/bitstream: Consistently treat symbol as VLC_TYPE

If a static VLC table gets initialized a second time (or concurrently by
two threads) and if said VLC table uses symbols that have the sign bit
of VLC_TYPE (a typedef for int16_t) set, initializing the VLC fails. The
reason is that the type of the symbol in the temporary array is an
uint16_t and so comparing it to the symbol read from the VLC table will
fail, because only the lower 16bits coincide. Said failure triggers an
assert.

Reviewed-by: Lynne <dev@lynne.ee>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/bitstream.c

@@ -129,7 +129,7 @@ static int alloc_table(VLC *vlc, int size, int use_static)
 
 typedef struct VLCcode {
     uint8_t bits;
-    uint16_t symbol;
+    VLC_TYPE symbol;
     /** codeword, with the first bit-to-be-read in the msb
      * (even if intended for a little-endian bitstream reader) */
     uint32_t code;