fix test failures

mysys/my_pread.c:
  don't set errno without a real error

mysys/my_pread.c

@@ -66,7 +66,8 @@ uint my_pread(File Filedes, byte *Buffer, uint Count, my_off_t offset,
 
     pthread_mutex_unlock(&my_file_info[Filedes].mutex);
 #else
-    error=((readbytes = (uint) pread(Filedes, Buffer, Count, offset)) != Count);
+    if ((error= ((readbytes =
+                  (uint) pread(Filedes, Buffer, Count, offset)) != Count)))
       my_errno= errno;
 #endif
     if (error || readbytes != Count)