s390/con3270: testing return kzalloc retval

Return value from kzalloc is not tested and
using a null pointer would lead to crash.

Even if this should not happen at this moment, we
may let the system decide if there is a better choice.

Signed-off-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>