This was uninitialized, showing garbage in the debugger

Bug: b/145351270
Change-Id: I952faabed4b1cf8b61dc4b9a5deb8c7a4e59db90
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/48688Reviewed-by: Alexis Hétu <sugoi@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>