trace: Fix void pointer arithmetic

The arithmetic on void pointer exists in trace routine. On GNU C, it
works because void pointer size is 1, but not all compilers behave like
this. So this patch specifies the size of the pointer.

src/utils/os_unix.c

@@ -370,7 +370,7 @@ void * os_realloc(void *ptr, size_t size)
 	if (ptr == NULL)
 		return os_malloc(size);
 
-	a = ptr - sizeof(*a);
+	a = (struct os_alloc_trace *) ptr - 1;
 	if (a->magic != ALLOC_MAGIC) {
 		wpa_printf(MSG_INFO, "REALLOC[%p]: invalid magic 0x%x%s",
 			   a, a->magic,
@@ -396,7 +396,7 @@ void os_free(void *ptr)
 
 	if (ptr == NULL)
 		return;
-	a = ptr - sizeof(*a);
+	a = (struct os_alloc_trace *) ptr - 1;
 	if (a->magic != ALLOC_MAGIC) {
 		wpa_printf(MSG_INFO, "FREE[%p]: invalid magic 0x%x%s",
 			   a, a->magic,