Crash after enabling a Texture pack with a ui/start_screen.json

Verification builds:
#332621

Summary:
When enabling a texture pack containing a ui/start_screen.json, the game crashes directly after returning to start screen after closing settings screen

Impact:
Game is crashing when enabling the TexturePack. It is a critical issue.

Repro Steps:
1. Enable PureBDcraft texture pack
2. Close settings to go back to start screen
3. Read the crash dialog

Observed Results:
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Assert Failure - ctrl+C to copy this message to the clipboard!
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Assertion failed: Invalid texture coordinates
Condition is false: coord.x >= 0.f && coord.x <= 1.f && coord.y >= 0.f && coord.y <= 1.f
Function: Tessellator::_tex in c:\DarwinWork\10\s\handheld\project\VS2015\Minecraft\Minecraft.Client\..\..\..\..\src-client\common\client\renderer\renderer\Tessellator.cpp @ 157

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OK
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Expected Results:
No crash (did not crash in build one week later)

Screenshots/Videos attached: No

Notes:
No matter what is customized in ui/start_screen.json, the game crashes at first texture pack activation when returning from settings screen to start screen. Game no longer crashes after