expand_string_buf, switch (*p), default case

Hello,
In `expand.c::expand_string_buf` there is a `switch (*p)` and in its
default case, there is
```
if (ISSPACE (p[-1]))
    break;
```
It seems to me that, at that point in the code, `p[-1]` is always `$` so
the check seems to be superfluous. Running `make check` (with coverage on)
hits `if (ISSPACE (p[-1]))` 2020 times but never enters. Am I missing
something?

Thanks,
Dimitar