Validating UsdCollectionAPI before accessing in UsdShadeMaterialBindingAPI::ComputeBoundMaterial

[sirpalee]

Description of Change(s)

Validating UsdCollectionAPI before accessing in UsdShadeMaterialBindingAPI::ComputeBoundMaterial.

This is a quick proposal to fix #2500 , but I'm unsure if this is the right place to do so. I.e. should we validate collections earlier, like in UsdShadeMaterialBindingAPI::_GetCollectionBindings or a similar place?

Fixes Issue(s)

• UsdShadeMaterialBindingAPI crashes if collection points to a non-existing property #2500

• I have verified that all unit tests pass with the proposed changes

• I have submitted a signed Contributor License Agreement

[jesschimein]

Filed as internal issue #USD-8440