Panic if free_blocks_count is invalid in malloc()

In malloc(), free_blocks_count was decremented only if it was greater
than zero. However, once a usable block has been found,
free_blocks_count should never be zero. If this condition occurs, it
indicates a fatal inconsistency in allocator state.

Replace the conditional decrement with a check that panics with
ERR_HEAP_CORRUPT when free_blocks_count <= 0, ensuring the kernel halts
on heap accounting corruption.

Author: visitorckw

lib/malloc.c

@@ -177,8 +177,11 @@ void *malloc(uint32_t size)
             }
 
             MARK_USED(p);
-            if (free_blocks_count > 0)
-                free_blocks_count--;
+            if (unlikely(free_blocks_count <= 0)) {
+                panic(ERR_HEAP_CORRUPT);
+                return NULL;
+            }
+            free_blocks_count--;
 
             CRITICAL_LEAVE();
             return (void *) (p + 1);