qga: Fix qmp_guest_get_memory_blocks() error handling

The Error ** argument must be NULL, &error_abort, &error_fatal, or a
pointer to a variable containing NULL.  Passing an argument of the
latter kind twice without clearing it in between is wrong: if the
first call sets an error, it no longer points to NULL for the second
call.

qmp_guest_get_memory_blocks() passes &local_err to
transfer_memory_block() in a loop.  If this fails in more than one
iteration, it can trip error_setv()'s assertion.

Fix it to break the loop.

Cc: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200422130719.28225-14-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

qga/commands-posix.c

@@ -2518,6 +2518,9 @@ GuestMemoryBlockList *qmp_guest_get_memory_blocks(Error **errp)
         mem_blk->phys_index = strtoul(&de->d_name[6], NULL, 10);
         mem_blk->has_can_offline = true; /* lolspeak ftw */
         transfer_memory_block(mem_blk, true, NULL, &local_err);
+        if (local_err) {
+            break;
+        }
 
         entry = g_malloc0(sizeof *entry);
         entry->value = mem_blk;