runtime/pprof/internal/profile: use idiomatic swapping

gogrep found only one such case with the pattern below:

	$tmp := $x; $x = $y; $y = $tmp

R=1.13

Change-Id: I6e46fb5ef2887f24fa9fc451323a8cef272e2886
Reviewed-on: https://go-review.googlesource.com/c/151200


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>