Fix segmentation fault of using --cache option without redis server

[marktwtn]

When running the tangle-accelerator with the command bazel run //accelerator -- --cache , the segmentation fault would occur if the Redis server is not activated.

Error message:

INFO: Analyzed target //accelerator:accelerator (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
Target //accelerator:accelerator up-to-date:
  bazel-bin/accelerator/accelerator
INFO: Elapsed time: 0.243s, Critical Path: 0.00s
INFO: 0 processes.
INFO: Build completed successfully, 1 total action
INFO: Build completed successfully, 1 total action
2020-03-24 22:05:31:         config:   INFO: [ta_core_default_init : 209] enable logger config.
2020-03-24 22:05:31:         config:   INFO: [ta_core_default_init : 219] Initializing TA information
2020-03-24 22:05:31:         config:   INFO: [ta_core_default_init : 232] Initializing Redis information
2020-03-24 22:05:31:         config:   INFO: [ta_core_default_init : 237] Initializing IRI configuration
2020-03-24 22:05:31:         config:   INFO: [ta_core_default_init : 245] Initializing IRI connection
2020-03-24 22:05:31:   cclient_core:   INFO: [logger_init_client_core:16] enable logger cclient_core.
2020-03-24 22:05:31:json_serializer:   INFO: [logger_init_json_serializer:16] enable logger json_serializer.
2020-03-24 22:05:31:cclient_extended:   INFO: [logger_init_client_extended:16] enable logger cclient_extended.
2020-03-24 22:05:31:         config:   INFO: [ta_core_set : 417] Initializing PoW implementation context
2020-03-24 22:05:31:         config:   INFO: [ta_core_set : 420] Initializing cache state
fish: 'bazel run //accelerator -- --ca…' terminated by signal SIGSEGV (Address boundary error)

The debugging command:

# build debug version
$ bazel build -c dbg //accelerator

# run with gdb
$ gdb --args bazel-bin/accelerator/accelerator