hmp: Guard against misuse of hmp_handle_error()

Null errp argument makes no sense.  Assert it's not null, to make this
explicit, and guard against misuse.  All current callers pass non-null
errp.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

hmp.c

@@ -28,7 +28,8 @@
 
 static void hmp_handle_error(Monitor *mon, Error **errp)
 {
-    if (error_is_set(errp)) {
+    assert(errp);
+    if (*errp) {
         monitor_printf(mon, "%s\n", error_get_pretty(*errp));
         error_free(*errp);
     }