segment: Fix behavior when #! line is not new-line terminated.

segment: Fix behavior when #! line is not new-line terminated.

The code here is supposed to maintain a invariant that, when it returns a
nonnegative value, it initializes *type, but it failed to do that when a
#! line did not end in a new-line.  This fixes the problem.

Bug #55101.
Thanks for Friedrich Beckmann for narrowing down the problem.
Found by the Debian buildd: https://buildd.debian.org/status/fetch.php?pkg=pspp&arch=arm64&ver=1.2.0-1&stamp=1543183214&raw=0