engines: check options before dereference

engines: check options before dereference

If FIO_OPT_STR_STORE option not provided it is initialized with NULL value, but
there are many places which assumes that is may be empty string
For example, commands below endup with null pointer dereference
fio  --name=test --ioengine=e4engine --size=1M
fio  --name=test --ioengine=rdma --port=1234 --size=1M