…when checking argument passing

The existing code was simply wrong: we cannot ever use the result
signature parameter list (rsig.params) if sigParams was adjusted
for variadic functions. If it was adjusted, we always must either
use sigParams or its separately instantiated version.

In the condition "n > 0 && adjusted", the "n > 0" should have
been in either of the respective "if statement" branches.

Simplified the code by merging with the result signature parameter
update.

Fixes #61932.

Change-Id: I5d39bc8bbc4dd85c7c985055d29532b4b176955e
Reviewed-on: https://go-review.googlesource.com/c/go/+/519456
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/519417
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>