[TF2] Fix crash when opening the loadout with random class selected

[SanyaKor]

Summary

Fixes a client crash that could occur when selecting the Random class and opening the loadout at the same time.

Cause

TF_CLASS_RANDOM was used as an index into g_VisibleLoadoutSlotsPerClass, even though Random is not a valid player class and has no loadout.

Changes

• Validate m_iCurrentClassIndex before accessing g_VisibleLoadoutSlotsPerClass.
• Ignore the openloadout command when no valid player class is selected.