privoxy-log-parser: Fix an 'uninitialized value' warning

privoxy-log-parser: Fix an 'uninitialized value' warning

... when generating statistics for a log file without response headers.

While privoxy-log-parser was supposed to detect this already,
the check was flawed and the message the user didn't see was
somewhat confusing anyway.

Now the message is less confusing, more helpful and actually printed.

Reported by: Robert Klemme