Compressible solver: correct initial Courant-number calculation

applications/solvers/combustion/reactingFoam/reactingFoam.C

...ctingFoam/rhoReactingBuoyantFoam/rhoReactingBuoyantFoam.C

...combustion/reactingFoam/rhoReactingFoam/rhoReactingFoam.C

...essible/rhoPimpleFoam/rhoPimpleDyMFoam/rhoPimpleDyMFoam.C

...ations/solvers/compressible/rhoPimpleFoam/rhoPimpleFoam.C

...olvers/compressible/sonicFoam/sonicDyMFoam/sonicDyMFoam.C

.../solvers/lagrangian/coalChemistryFoam/coalChemistryFoam.C

...olvers/lagrangian/reactingParcelFoam/reactingParcelFoam.C

...ions/solvers/multiphase/cavitatingFoam/setInitialDeltaT.H