builtin: Fix undefined behaviour when allocating empty structs (#4088)

vlib/builtin/builtin.v

@@ -177,6 +177,9 @@ pub fn free(ptr voidptr) {
 }
 
 pub fn memdup(src voidptr, sz int) voidptr {
+	if sz == 0 {
+		return vcalloc(1)
+	}
 	mem := malloc(sz)
 	return C.memcpy(mem, src, sz)
 }

vlib/compiler/tests/struct_test.v

@@ -1,10 +1,10 @@
-struct A{
+struct A {
 mut:
 	val int
 	nums []int
 }
 
-struct B{
+struct B {
 mut:
 	a A
 }
@@ -26,6 +26,9 @@ struct Foo {
 	@type string
 }
 
+struct Empty {
+}
+
 //We need to make sure that this compiles with all the reserved names.
 struct ReservedKeywords {
 	delete   int
@@ -55,6 +58,12 @@ struct ReservedKeywords {
 	while    int
 }
 
+fn test_empty_struct() {
+	d := &Empty{}
+	println(d) // != voidptr(0)
+	println(sizeof(Empty)) // == 0
+}
+
 fn test_struct_levels() {
 	mut c := C{}
 	assert c.nums.len == 0