Non-zero offset added to nullptr causes ASAN errors

[aabdolrashidi]

The Chromium rolls into ANGLE (latest roll) have started failing many tests on Linux when using UBSan/ASAN settings. (Related issue)

The error seems to come from multiple locations in two test files where a non-zero offset is added to a potentially nullptr value, which is undefined behavior:

• modules/glshared/glsDrawTest.cpp
  • Lines 1491, 1498, and 1510 (related to vertexAttrib*Pointer())
  • Line 3789 (indexPointer)
• framework/common/tcuTextureUtil.cpp
  • Lines 1560 and 1565 (return value of addOffset())

Using the following equivalent for each case should fix this:

basePointer + offset --> (basePointer) ? basePointer + offset : reinterpret_cast<T*>(offset)