Bug#14838882: INIT_IO_CACHE() HAS SUSPICIOUS MEMSET()

The problem was that init_io_cache() contained a memset() that with
SAFE_MUTEX was used to clear a mutex - IO_CACHE::append_buffer_lock.
However the size of memory cleared was the size of the IO_CACHE pointer
rather than the size of the IO_CACHE::append_buffer_lock mutex.

This patch fixes the problem by clearing the appropriate number
of bytes.

Author: jhauglid

mysys/mf_iocache.c

@@ -259,7 +259,7 @@ int init_io_cache(IO_CACHE *info, File file, size_t cachesize,
   else
   {
     /* Clear mutex so that safe_mutex will notice that it's not initialized */
-    memset(&info->append_buffer_lock, 0, sizeof(info));
+    memset(&info->append_buffer_lock, 0, sizeof(info->append_buffer_lock));
   }
 #endif