Don't log warnings if no block-type handlers are registered.

Check whether the block-type hash table exists before trynig to look
up anything in it.

Change-Id: I2c5badaed4f7c7dfb5caa486a8aacf3bf326cd09
Reviewed-on: https://code.wireshark.org/review/4690
Reviewed-by: Guy Harris <guy@alum.mit.edu>