Single Letter path crashes par2

[lkydev]

I stumbled across the following bug while creating a parity set.

Par2cmdline 0.8.1
FreeBSD 13.0 (x86)
Debian 11.0 (x86)
Debian 11.0 (x64) on WSL1 and WSL2.

# create a file named 'a' (a single letter)
$ dd if=/dev/urandom of=a bs=10000 count=100

# create a parity set
$ par2 c -s10000 -c100 test.par2 a

Block size: 10000
Source file count: 1
Source block count: 1000
Recovery block count: 100
Recovery file count: 7

Opening: a
terminate called after throwing an instance of 'std::out_of_range'
  what():  basic_string::at: __n (which is 1) >= this->size() (which is 1)
Aborted (core dumped)

[animetosho]

See #149