migration: fix -Werror=maybe-uninitialized false-positive

../migration/ram.c:1873:23: error: ‘dirty’ may be used uninitialized [-Werror=maybe-uninitialized]

When 'block' != NULL, 'dirty' is initialized.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>

migration/ram.c

@@ -1793,7 +1793,7 @@ static bool get_queued_page(RAMState *rs, PageSearchStatus *pss)
 {
     RAMBlock  *block;
     ram_addr_t offset;
-    bool dirty;
+    bool dirty = false;
 
     do {
         block = unqueue_page(rs, &offset);