MINOR: chunk: New function free_trash_buffers()

This new function is meant to be called in the general deinit phase,
to free those two internal chunks.
diff --git a/src/chunk.c b/src/chunk.c
index 3c1cfdf..1359adc 100644
--- a/src/chunk.c
+++ b/src/chunk.c
@@ -66,6 +66,17 @@
 }
 
 /*
+ * free the trash buffers
+ */
+void free_trash_buffers(void)
+{
+	free(trash_buf2);
+	free(trash_buf1);
+	trash_buf2 = NULL;
+	trash_buf1 = NULL;
+}
+
+/*
  * Does an snprintf() at the beginning of chunk <chk>, respecting the limit of
  * at most chk->size chars. If the chk->len is over, nothing is added. Returns
  * the new chunk size, or < 0 in case of failure.