Fix null dereference in OFFSET macro
Fix null dereference in OFFSET macro

Applied same fix as done in 0e6a044 for the same macro in
src/Common/Types.hpp (including copying the comment verbatim).

Change-Id: I822db18f19a5473d9cf56ab57339ae91e96f458e
Bug: b/155089897
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/44508
Reviewed-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>
Kokoro-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Tested-by: James Price <a76907c12d6081e0086dde10f0b94b867c877a2c@google.com>

google/swiftshader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader