[RFC] Fix newline substitution causing abort

Fixes #1529.

As explained there, this is one of two possible solutions, that should have to be discussed. 
But, in the meantime, it's requested for this to be merged so that master doesn't abort in that case.
Sorry to harp on this, but it's more confusing now, IMO. I would be fine with just your original version, with the operators on the aligned column.

```c
    linenr_T joined_lines_count = eap->line2 < curbuf->b_ml.ml_line_count
                                  ? eap->line2 - eap->line1 + 2
                                  : eap->line2 - eap->line1 + 1;
```