Improve SSL error message

Log the type of the DCB to help figure out where the error came from.

server/core/dcb.cc

@@ -828,9 +828,9 @@ static int dcb_log_errors_SSL(DCB* dcb, int ret)
     }
     if (ret || ssl_errno)
     {
-        MXS_ERROR("SSL operation failed, dcb %p in state "
+        MXS_ERROR("SSL operation failed, %s in state "
                   "%s fd %d return code %d. More details may follow.",
-                  dcb,
+                  dcb->type(),
                   STRDCBSTATE(dcb->state),
                   dcb->fd,
                   ret);