x86: Fix x86_cpu_new() error handling

The Error ** argument must be NULL, &error_abort, &error_fatal, or a
pointer to a variable containing NULL.  Passing an argument of the
latter kind twice without clearing it in between is wrong: if the
first call sets an error, it no longer points to NULL for the second
call.

x86_cpu_new() is wrong that way: it passes &local_err to
object_property_set_uint() without checking it, and then to
qdev_realize().  If both fail, we'll trip error_setv()'s assertion.
To assess the bug's impact, we'd need to figure out how to make both
calls fail.  Too much work for ignorant me, sorry.

Fix by checking for failure right away.

Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200630090351.1247703-21-armbru@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

hw/i386/x86.c

@@ -118,14 +118,16 @@ uint32_t x86_cpu_apic_id_from_index(X86MachineState *x86ms,
 
 void x86_cpu_new(X86MachineState *x86ms, int64_t apic_id, Error **errp)
 {
-    Object *cpu = NULL;
     Error *local_err = NULL;
-
-    cpu = object_new(MACHINE(x86ms)->cpu_type);
+    Object *cpu = object_new(MACHINE(x86ms)->cpu_type);
 
     object_property_set_uint(cpu, apic_id, "apic-id", &local_err);
+    if (local_err) {
+        goto out;
+    }
     qdev_realize(DEVICE(cpu), NULL, &local_err);
 
+out:
     object_unref(cpu);
     error_propagate(errp, local_err);
 }