I guess I'd vote for changing the code to be

        sys = malloc(strlen(editorName) + strlen(fname) + 10 + 1);
        if (!sys)
                return false;
        sprintf(sys, "exec '%s' '%s'", editorName, fname);

(note the added quotes to provide a little protection against spaces
and such).  Then it's perfectly obvious what the calculation is doing.
I don't care about wasting 20-some bytes, but confusing readers of the
code is worth avoiding.

                        regards, tom lane