Fix mulfac corner case

[mloubout]

Lead to issues with very small wavefields (i.e elastic) with an Inf mulfact that cannot be decompressed with

Error! Decompress: nx, ny, nz do not match!
nx=540, ny=141, nz=141, nx_check=0, ny_check=0, nz_check=0

I think @jkwashbourne-oss had seen it too in some random runs.