vl: catch [accel] entry without accelerator

Avoid a SIGSEGV and return an error instead.

Reported-by: Thomas Huth <thuth@redhat.com>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1439
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

softmmu/vl.c

@@ -2204,14 +2204,18 @@ static int do_configure_accelerator(void *opaque, QemuOpts *opts, Error **errp)
     int ret;
     bool qtest_with_kvm;
 
+    if (!acc) {
+        error_setg(errp, QERR_MISSING_PARAMETER, "accel");
+        goto bad;
+    }
+
     qtest_with_kvm = g_str_equal(acc, "kvm") && qtest_chrdev != NULL;
 
     if (!ac) {
-        *p_init_failed = true;
         if (!qtest_with_kvm) {
             error_report("invalid accelerator %s", acc);
         }
-        return 0;
+        goto bad;
     }
     accel = ACCEL(object_new_with_class(OBJECT_CLASS(ac)));
     object_apply_compat_props(OBJECT(accel));
@@ -2221,14 +2225,17 @@ static int do_configure_accelerator(void *opaque, QemuOpts *opts, Error **errp)
 
     ret = accel_init_machine(accel, current_machine);
     if (ret < 0) {
-        *p_init_failed = true;
         if (!qtest_with_kvm || ret != -ENOENT) {
             error_report("failed to initialize %s: %s", acc, strerror(-ret));
         }
-        return 0;
+        goto bad;
     }
 
     return 1;
+
+bad:
+    *p_init_failed = true;
+    return 0;
 }
 
 static void configure_accelerators(const char *progname)