cmd/compile: fix wrong check for b.Controls in isBlockMultiValueExit

b.Controls has type [2]*Value, thus len(b.Controls) > 0 is always true.
The right check should be b.Controls[0] != nil, though, this is also
always true, since when we always set control value for BlockRet and
BlockRetJmp when state.exit is called.

Though checkFunc also checks for nil control value of ret/retjmp, but
it happens later after expand_calls pass, so better to be defensive
here, just in case.

Change-Id: Ie4a292a3494dfbf5e6d872cde498703023b84d00
Reviewed-on: https://go-review.googlesource.com/c/go/+/345433


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>