Fix warning treated as error that loop only iterates once.

Bug: b/148401179
Change-Id: I8caf237ede034ba6802a6fbc8167140b27cc13cb
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/44548Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>