Fix null ref exception in LookupType5Format2SubTable

[JimBobSquarePants]

Prerequisites

• [x] I have written a descriptive pull-request title
• [x] I have verified that there are no overlapping pull-requests open
• [x] I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules :cop:.
• [x] I have provided test coverage for my change (where applicable)

Description

Fixes #364 cc @IshmaZX82

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (e5a0f4a) 86% compared to head (3ef36ae) 86%.

Files	Patch %	Lines
.../Tables/AdvancedTypographic/Shapers/IndicShaper.cs	50%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #365 +/- ## ===================================== Coverage 86% 86% ===================================== Files 238 238 Lines 15182 15184 +2 Branches 2100 2101 +1 ===================================== + Hits 13085 13088 +3 Misses 1613 1613 + Partials 484 483 -1 ``` | [Flag](https://app.codecov.io/gh/SixLabors/Fonts/pull/365/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SixLabors) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/SixLabors/Fonts/pull/365/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SixLabors) | `86% <80%> (+<1%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SixLabors#carryforward-flags-in-the-pull-request-comment) to find out more.

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