tests: lavfi/drawutils: use av_err2str

There is no need to explicitly specify the buffer, as it
is only ever passed to printf, so av_err2str can be used.

libavfilter/tests/drawutils.c

@@ -37,9 +37,7 @@ int main(void)
                (int)(16 - strlen(desc->name)), "");
         r = ff_draw_init(&draw, f, 0);
         if (r < 0) {
-            char buf[128];
-            av_strerror(r, buf, sizeof(buf));
-            printf("no: %s\n", buf);
+            printf("no: %s\n", av_err2str(r));
             continue;
         }
         ff_draw_color(&draw, &color, (uint8_t[]) { 1, 0, 0, 1 });