Whichever direction was chosen as the 'major' one, a division by zero
occurred when trying to project to its corresponding face. Prevent it by
ensuring 'M' is greater than zero.

Bug: b/132981873
Test: dEQP-VK.glsl.opaque_type_indexing.sampler.const_literal.compute.samplercube
Change-Id: I71c25be4d8e17409cc9bf32a2f6180db9a948d34
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/31729
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>