For an unknown reason, Logger died few days ago while writing the history. The
file mhf_1489577446_20170315_system.dat was created, but was empty.

When trying to restart Logger, I would get a seg fault without any special error
message.

I tracked the issue to the "read_file_schema" function in history_schema.cxx

* L4731, a pointer to HsFileSchema *s is declared.
* L4747, We enter a while(1) loop.
* L4749, get char on the filename.
In our case, the file was empty, so the variable "b" gets NULL and the loop breaks.

Problem: the memory allocation for "s" is later in the loop, L4768.
Upon exiting the loop, L4854, we try to access record_size on a NULL pointer ==>
SegFault. 

It would be nice to at least have a message before breaking the loop...