migration: fix uninitialized variable warning in migrate_send_rp_req_pages()

migration: fix uninitialized variable warning in migrate_send_rp_req_pages()

After the WITH_QEMU_LOCK_GUARD macro is added, the compiler cannot identify
 that the statements in the macro must be executed. As a result, some variables
 assignment statements in the macro may be considered as unexecuted by the compiler.

When the -Wmaybe-uninitialized capability is enabled on GCC9,the compiler showed warning:
migration/migration.c: In function ‘migrate_send_rp_req_pages’:
migration/migration.c:384:8: warning: ‘received’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 384 |     if (received) {
     |        ^

Add a default value for 'received' to prevented the warning.

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Chen Qun <kuhn.chenqun@huawei.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20201111142203.2359370-6-kuhn.chenqun@huawei.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>