xodtemplate: Avoid reading uninitialized memory

Let's say a config file contains something like

  define service {
      service_description foo
      host_name
  }

We first tokenize this on newlines, and separate them with newlines.
After that, we tokenize the lines on the first whitespace after
the first nonwhitespace character - for the host_name line, this will fail.

However, we'd think that we'd been successful, and started to insert
whatever random garbage was in the memory after this key (which, due to
strip()ing, was probably the end of the key name, assuming we had
indentation in place).

In this case, we wouldn't start, because looking up a host named "ame"
or so would most likely fail and a confused user would remain, wondering
who told Nagios to look for that. In other cases, who knows?

Signed-off-by: Robin Sonefors <robin.sonefors@op5.com>