nutUBlendedWallFunction causes floating point exeption

new nutUBlendedWallFunction may cause a floating point exeption.

Using nutUBlendedWallFunction in simpleFoam/pitzDaily tutorial case will reproduce this error. Zero gradient at the initial state leads to yPlus of zero.

Changing the code from

scalar yPlus = y[facei]*ut/nuw[facei];

to

scalar yPlus = y[facei]*ut/nuw[facei] + VSMALL;

will prevent the problems (division by zero and logarithm of zero).