crypto/scheduler: set null pointer after freeing

crypto/scheduler: set null pointer after freeing

When freeing memory, pointers should be set to NULL,
to avoid memory corruption/segmentation faults.

Fixes: 31439ee72b2c ("crypto/scheduler: add API implementations")
Fixes: 50e14527b9d1 ("crypto/scheduler: improve parameters parsing")
Fixes: 57523e682bb7 ("crypto/scheduler: register operation functions")
Fixes: a783aa634410 ("crypto/scheduler: add packet size based mode")
Fixes: 4c07e0552f0a ("crypto/scheduler: add multicore scheduling mode")
Cc: stable@dpdk.org
Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: Fan Zhang <roy.fan.zhang@intel.com>