Don't read potentially uninitalized memory if gethostname() failed

Don't read potentially uninitalized memory if gethostname() failed

If the buffer is smaller than `HOST_NAME_MAX` (64 on Linux but up to
255 bytes in POSIX, which FreeBSD, MacOS etc honor) gethostname()
might return -1 without null-terminating the buffer, causing an
out-of-bounds read.
As we look for the first '.' using `strchr()`, replacing it with a
null byte, we also have a one-byte out-of-bounds write which might
result in a crash or, albeit very unlikely, arbitrary code execution.