record: Fix out of bounds access in SwapCreateRegister()

ZDI-CAN-14952, CVE-2021-4011

This vulnerability was discovered and the fix was suggested by:
Jan-Niklas Sohn working with Trend Micro Zero Day Initiative

Signed-off-by: Povilas Kanapickas <povilas@radix.lt>
(cherry picked from commit e56f61c79f)

record/record.c

@@ -2515,8 +2515,8 @@ SwapCreateRegister(ClientPtr client, xRecordRegisterClientsReq * stuff)
         swapl(pClientID);
     }
     if (stuff->nRanges >
-        client->req_len - bytes_to_int32(sz_xRecordRegisterClientsReq)
-        - stuff->nClients)
+        (client->req_len - bytes_to_int32(sz_xRecordRegisterClientsReq)
+        - stuff->nClients) / bytes_to_int32(sz_xRecordRange))
         return BadLength;
     RecordSwapRanges((xRecordRange *) pClientID, stuff->nRanges);
     return Success;