Fix GTIL if input does not have sufficient number of columns

[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (cb1aa86) 84.24% compared to head (fbbaf6a) 84.40%.

Files	Patch %	Lines
python/treelite/gtil/gtil.py	33.33%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## mainline #550 +/- ## ============================================ + Coverage 84.24% 84.40% +0.15% ============================================ Files 71 71 Lines 6068 6071 +3 Branches 510 510 ============================================ + Hits 5112 5124 +12 + Misses 956 947 -9 ```

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