all: disallow pointer arithmetic for AssignStmt, PostfixExpr outside unsafe{} (#5581)

vlib/builtin/builtin.v

@@ -137,7 +137,9 @@ pub fn malloc(n int) byteptr {
 	}
 	$if prealloc {
 		res := g_m2_ptr
-		g_m2_ptr += n
+		unsafe {
+			g_m2_ptr += n
+		}
 		nr_mallocs++
 		return res
 	} $else {