Fix config file parsing

[kephir4eg]

Motivating case:

$ ./mergerfs -o config=any.conf aaa testmnt
terminate called after throwing an instance of 'std::out_of_range'
  what():  basic_string::substr: __pos (which is 2) > this->size() (which is 0)
Aborted (core dumped)

The content of the config file is irrelevant, the relevant stack is:

#10 0x000000000040cc4e in Branches::Impl::from_string (this=0x680540, s_="") at src/branches.cpp:316
#11 0x000000000040d8e7 in Branches::from_string (this=0x7fffffffc500, str_="") at src/branches.cpp:381
#12 0x000000000042da04 in Config::operator= (this=0x7fffffffc4d0, cfg_=...) at src/config.cpp:193
#13 0x000000000042ecab in Config::from_stream (this=0x666d80 <Config::_singleton>, istrm_=..., errs_=0x7fffffffd4b0) at src/config.cpp:302
#14 0x000000000042f93e in Config::from_file (this=0x666d80 <Config::_singleton>, filepath_="empty.conf", errs_=0x7fffffffd4b0) at src/config.cpp:341

The error happens while trying to assign the initial config object to the new instance in Config::from_stream. Empty branch (initial value) cannot be parsed by from_branch(). In fact having branch= line in the config would have the same result.

I also created a test, which explains the issue.

[trapexit]

Thanks. I knew that issue was there but hadn't gotten around to addressing it.