multi: clarify condition in curl_multi_wait

`if(nfds || extra_nfds) {` is followed by `malloc(nfds * ...)`.

If `extra_fs` could be non-zero when `nfds` was zero, then we have
`malloc(0)` which is allowed to return `NULL`. But, malloc returning
NULL can be confusing. In this code, the next line would treat the NULL
as an allocation failure.

It turns out, if `nfds` is zero then `extra_nfds` must also be zero.
The final value of `nfds` includes `extra_nfds`.  So the test for
`extra_nfds` is redundant.  It can only confuse the reader.

Closes #1439