Wconversion: fix #1578

In fact, this entire things might be written differently. It appears to be some concatenation:

```c
char *s = xmalloc(4 + cmdlen + 2 + strlen(mesg));
val = stpcpy("):", stpcpy(cmdname, stpcpy(s, "Vim(")));
```

Untested, just some scribbling. The only thing that bothers me is that it looks inverted. Can be solved by:

```c
char *s = xmalloc(4 + cmdlen + 2 + strlen(mesg));
val = stpcpy(s, "Vim("));
val = stpcpy(val, cmdname);
val = stpcpy(val, "):");
```

Which is a bit longer but better illustrates what's going on.