avcodec/a64multienc: Don't use static buffers, fix potential races

render_charset() used static buffers that are always completely
initialized before every use, so that it is unnecessary for the
values in these arrays to be kept after leaving the function.
Given that this is not only unnecessary, but harmful due to the
possibility of data races if several instances of a64multi/a64multi5
run simultaneously these buffers have been replaced by ordinary buffers
on the stack (they are small enough for this).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
(cherry picked from commit 0ca09335aa)

libavcodec/a64multienc.c

@@ -107,13 +107,16 @@ static void render_charset(AVCodecContext *avctx, uint8_t *charset,
     uint8_t pix;
     int lowdiff, highdiff;
     int *best_cb = c->mc_best_cb;
-    static uint8_t index1[256];
-    static uint8_t index2[256];
-    static uint8_t dither[256];
+    uint8_t index1[256];
+    uint8_t index2[256];
+    uint8_t dither[256];
     int i;
     int distance;
 
-    /* generate lookup-tables for dither and index before looping */
+    /* Generate lookup-tables for dither and index before looping.
+     * This code relies on c->mc_luma_vals[c->mc_pal_size - 1] being
+     * the maximum of all the mc_luma_vals values and on the minimum
+     * being zero; this ensures that dither is properly initialized. */
     i = 0;
     for (a=0; a < 256; a++) {
         if(i < c->mc_pal_size -1 && a == c->mc_luma_vals[i + 1]) {