Using positionCount resizes the line renderer position array, but the values get often filled with random or erroneous data.

Repro steps:
1. Open Scene1 from the attached project.
2. Press play
3. Observe the Line Renderer's positions data getting random (and sometimes NaN) values and the console spamming errors (image attached)

Expected outcome: new positions to be set to Vector3.zero

Notes:
- not a regression, happens in 5.6.0p1 as well