add nilassign linter (#2131)

Co-authored-by: Fernandez Ludovic <ldez@users.noreply.github.com>

test/testdata/nilassign.go

@@ -0,0 +1,28 @@
+//args: -Enilassign
+package testdata
+
+var num = 1
+
+func pvar() {
+	var ii *int
+	*ii = 1 // ERROR "this assignment occurs invalid memory address or nil pointer dereference"
+
+	var i *int
+	i = &num // OK
+	_ = i    // OK
+}
+
+func pstruct() {
+	n := new(Node)
+
+	*n.PVal = 1           // ERROR "this assignment occurs invalid memory address or nil pointer dereference"
+	*n.ChildNode.PVal = 1 // ERROR "this assignment occurs invalid memory address or nil pointer dereference"
+
+	n.ChildNode = &Node{PVal: &num} // OK
+	n.PVal = &num                   // OK
+}
+
+type Node struct {
+	PVal      *int
+	ChildNode *Node
+}