I am trying to replace few zeros in one of the columns of pandas dataframe with the difference between values in the previous two columns in the same row.

```
   A     B      C
0  10    12     -2
1  6     3       0
2  5     18      0
3  3     11     -8

```

I want to replace the zeros in column `C` with the difference between respective values in `A` and `B`. Replace only zeros not a non-zero value.