Sampling primvars is causing a crash when multiple threads are used to sync primitives

[sirpalee]

Describe the bug During investigating #72 we found a crash when sampling primvars (point positions). This seemed to happen randomly, and setting PXR_WORK_THREAD_LIMIT to 1 fixed the issue.

To Reproduce Steps to reproduce the behavior:

1. Load the point instance med city repeteadly
2. Observe a crash happening a 2-3 times out of 10.

Used Software Versions

• Arnold: 6.0.2.0
• USD: 0.19.11
• OS: Windows 10