block/qapi: Fix memory leak in qmp_query_blockstats()

For BlockBackends that are skipped in query-blockstats, we would leak
info since commit 567dcb31. Allocate info only later to avoid the memory
leak.

Fixes: CID 1394727
Cc: qemu-stable@nongnu.org
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NAlberto Garcia <berto@igalia.com>
(cherry picked from commit f62492bb)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

block/qapi.c

@@ -594,7 +594,7 @@ BlockStatsList *qmp_query_blockstats(bool has_query_nodes,
         }
     } else {
         for (blk = blk_all_next(NULL); blk; blk = blk_all_next(blk)) {
-            BlockStatsList *info = g_malloc0(sizeof(*info));
+            BlockStatsList *info;
             AioContext *ctx = blk_get_aio_context(blk);
             BlockStats *s;
             char *qdev;
@@ -619,6 +619,7 @@ BlockStatsList *qmp_query_blockstats(bool has_query_nodes,
             bdrv_query_blk_stats(s->stats, blk);
             aio_context_release(ctx);
 
+            info = g_malloc0(sizeof(*info));
             info->value = s;
             *p_next = info;
             p_next = &info->next;