The VaryingNames array has NumVaryings entries. But BufferStride is a small array of MAX_FEEDBACK_BUFFERS (4) entries. Programs with more than 4 varyings would read out of bounds.
Fixes valgrind errors in KHR-GL45.transform_feedback.linking_errors_test.
Fixes: 6d830940 glsl/shader_cache: Allow shader cache usage with transform feedback