hmp: Fix warning from smatch (wrong argument in function call)

Fix this warning:
hmp.c:414:38: warning: Using plain integer as NULL pointer

qmp_query_block expects a pointer argument, so passing false is wrong.

Cc: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

hmp.c

@@ -411,7 +411,7 @@ void hmp_info_block(Monitor *mon, const QDict *qdict)
 
     /* Print BlockBackend information */
     if (!nodes) {
-        block_list = qmp_query_block(false);
+        block_list = qmp_query_block(NULL);
     } else {
         block_list = NULL;
     }