fixed memheap size type.

src/memheap.c

@@ -600,11 +600,12 @@ static struct rt_memheap _heap;
 
 void rt_system_heap_init(void *begin_addr, void *end_addr)
 {
+    RT_ASSERT((rt_size_t)end_addr > (rt_size_t)begin_addr);
     /* initialize a default heap in the system */
     rt_memheap_init(&_heap,
                     "heap",
                     begin_addr,
-                    (rt_uint32_t)end_addr - (rt_uint32_t)begin_addr);
+                    (rt_size_t)end_addr - (rt_size_t)begin_addr);
 }
 
 void *rt_malloc(rt_size_t size)