QEMU in some arch will crash when executing -vga help command, because
there is no default vga model.  Add check to this case and avoid crash.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/978

Signed-off-by: Guo Zhi <qtxuning1999@sjtu.edu.cn>
---
 softmmu/vl.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/softmmu/vl.c b/softmmu/vl.c
index c2919579fd..a49e29312b 100644
--- a/softmmu/vl.c
+++ b/softmmu/vl.c
@@ -977,7 +977,8 @@ static void select_vgahw(const MachineClass *machine_class, const char *p)
 
             if (vga_interface_available(t) && ti->opt_name) {
                 printf("%-20s %s%s\n", ti->opt_name, ti->name ?: "",
-                       g_str_equal(ti->opt_name, def) ? " (default)" : "");
+                        (def && g_str_equal(ti->opt_name, def)) ?
+                        " (default)" : "");
             }
         }
         exit(0);
-- 
2.35.1