[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.
Style guide encourages operators on the aligned column (i.e., the left 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;
```

Alternative:

```c
    linenr_T joined_lines_count = eap->line2 - eap->line1 
                  + (eap->line2 < curbuf->b_ml.ml_line_count ? 2 : 1);
```