Server uses gethostname() for the default base name for pid/log files.
If a character is not representable in current ANSI encoding, gethostname
replaces it with question mark. Thus, generated log file name would also
contain a question mark. However, Windows forbids certain characters in
filenames, among them '?'.

This is described in MSDN article https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx
At attempts to create the file via freopen() fails, thus server would not
be able to start.

The fix is to verify hostname and fall back to  "mysql", if
invalid characters are found.