go.tools/cmd/vet: fix a panic on invalid AddInt

Ignore calls to various flavours of atomic.AddInt with a wrong
number of arguments.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://golang.org/cl/91370045

cmd/vet/atomic.go

@@ -44,6 +44,9 @@ func (f *File) checkAtomicAssignment(n *ast.AssignStmt) {
 // checkAtomicAddAssignment walks the atomic.Add* method calls checking for assigning the return value
 // to the same variable being used in the operation
 func (f *File) checkAtomicAddAssignment(left ast.Expr, call *ast.CallExpr) {
+	if len(call.Args) != 2 {
+		return
+	}
 	arg := call.Args[0]
 	broken := false
 

cmd/vet/testdata/atomic.go

@@ -38,4 +38,6 @@ func AtomicTests() {
 	ap := []*uint64{&au[0], &au[1]}
 	*ap[0] = atomic.AddUint64(ap[0], 1) // ERROR "direct assignment to atomic value"
 	*ap[1] = atomic.AddUint64(ap[0], 1)
+
+	x = atomic.AddUint64() // Used to make vet crash; now silently ignored.
 }