[Bug] Wrong MEM_POOL check in rt_smem_free

RT-Thread Version

master

Hardware Type/Architectures

all

Develop Toolchain

IAR

Describe the bug

Describe the bug
In rt_smem_free() (file: src/mem.c), the following assertion is used:
RT_ASSERT(MEM_POOL(&small_mem->heap_ptr[mem->next]) == small_mem);
This assertion fails incorrectly when freeing a block located near the heap end (heap_end).
At this point, the logic actually refers to the previous block (mem->prev), not mem->next.
Since the heap_end block is always marked as USED, MEM_POOL(...) does not equal small_mem, causing a false assertion failure.

How to reproduce

• Initialize rt_smem with a small heap.
• Allocate and free a block close to the heap_end.
• Observe assert failed inside rt_smem_free.

Expected behavior
Memory should be freed correctly without triggering a false assertion.

Root cause

• The assertion checks mem->next, which may point to the sentinel heap_end.
• heap_end is always USED, so MEM_POOL() fails.
• The check should instead validate mem->prev at this point.

Proposed fix
Change the assertion from mem->next to mem->prev.
This aligns the implementation with the original lwIP small memory allocator.

--- a/src/mem.c
+++ b/src/mem.c
@@ void rt_smem_free(void *rmem)
-    RT_ASSERT(MEM_POOL(&small_mem->heap_ptr[mem->next]) == small_mem);
+    RT_ASSERT(MEM_POOL(&small_mem->heap_ptr[mem->prev]) == small_mem);

Environment

• RT-Thread version: [master branch]
• Module: components src/mem.c
• Target: [all]

Additional context

• Verified with test cases: issue reproduced on small heap allocation/free.
• After patch, allocator works without false assertion.
• Behavior now matches lwIP mem_free() / plug_holes().

Other additional context

No response

Start in src/mem.c at rt_smem_free() and inspect the assertion involving small_mem->heap_ptr and the neighboring block pointers. Reproduce the issue with a small heap by allocating and freeing a block near heap_end; done means the free operation no longer triggers a false assertion and matches the described allocator behavior.