patch 9.2.0256: visual selection size not shown in showcmd during test

Problem:  The visual selection size is not displayed in the showcmd area
          when entering visual mode from a script or mapping, because
          char_avail() incorrectly reports input as pending. This causes
          test failure on CI with the ASAN CI runner.
Solution: Replace char_avail() with explicit checks for an empty stuff
          buffer, empty typeahead buffer, and not running a script
          (zeertzjq).

related: #19801
closes:  #19824

Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

Author: zeertzjq

src/normal.c

@@ -1616,7 +1616,8 @@ clear_showcmd(void)
     if (!p_sc)
 	return;
 
-    if (VIsual_active && !char_avail())
+    if (VIsual_active
+	&& stuff_empty() && typebuf.tb_len == 0 && !using_script())
     {
 	int		cursor_bot = LT_POS(VIsual, curwin->w_cursor);
 	long		lines;

src/testdir/test_visual.vim

@@ -1634,7 +1634,6 @@ func Test_visual_block_hl_with_autoselect()
   call writefile(lines, 'XTest_visual_block_autoselect', 'D')
 
   let buf = RunVimInTerminal('-S XTest_visual_block_autoselect', {'rows': 10})
-  call TermWait(buf)
   call VerifyScreenDump(buf, 'Test_visual_block_hl_with_autoselect_1', {})
   call term_sendkeys(buf, 'l')
   call VerifyScreenDump(buf, 'Test_visual_block_hl_with_autoselect_2', {})

src/version.c

@@ -734,6 +734,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    256,
 /**/
     255,
 /**/