Fix compile failure:

In function ‘void* memset(void*, int, size_t)’,
    inlined from ‘void Lifo_buffer::set_buffer_space(uchar*, uchar*)’ at sql_lifo_buffer.h:70:5,
    inlined from ‘int DsMrr_impl::dsmrr_init(handler*, RANGE_SEQ_IF*, void*, uint, uint, HANDLER_BUFFER*)’ at multi_range_read.cc:895:62:
/usr/include/i386-linux-gnu/bits/string3.h:82:32: error: call to ‘__warn_memset_zero_len’ declared with attribute warning: memset used with constant zero length parameter; this could be due to transposed parameters [-Werror]

It was intentional that the buffer is set to zero length there.

sql/sql_lifo_buffer.h

@@ -67,7 +67,8 @@ public:
   {
     start= start_arg;
     end= end_arg;
-    TRASH(start, end - start);
+    if (end != start)
+      TRASH(start, end - start);
     reset();
   }