Fix memset after GC

Author: osa1

rts/motoko-rts/src/gc.rs

@@ -484,7 +484,7 @@ pub unsafe extern "C" fn rust_collect_garbage() {
 
     // Reset scratch space
     memset(
-        begin_to_space,
+        new_hp,
         bytes_to_words(Bytes((end_to_space - new_hp) as u32)),
         0,
     );