patch 8.2.1608: Vim9: getchar() test fails with GUI

Problem:    Vim9: getchar() test fails with GUI.
Solution:   Avoid that getchar(0) gets stuck on K_IGNORE.
diff --git a/src/getchar.c b/src/getchar.c
index 4f5ea46..e42fc33 100644
--- a/src/getchar.c
+++ b/src/getchar.c
@@ -2044,10 +2044,12 @@
 	    // illegal argument or getchar(0) and no char avail: return zero
 	    n = 0;
 	else
-	    // getchar(0) and char avail: return char
-	    n = plain_vgetc();
+	    // getchar(0) and char avail() != NUL: get a character.
+	    // Note that vpeekc_any() returns K_SPECIAL for K_IGNORE.
+	    n = safe_vgetc();
 
-	if (n == K_IGNORE || n == K_MOUSEMOVE)
+	if (n == K_IGNORE || n == K_MOUSEMOVE
+		|| n == K_VER_SCROLLBAR || n == K_HOR_SCROLLBAR)
 	    continue;
 	break;
     }