The main problem was that lex_start() was forgotten to be called before processing
COM_REFRESH.

Another problem discovered was that if failures to flush the error log were not properly
handled, which resulted in the server crash.

The user-visible effect of these problems were:
  - if COM_REFRESH command was sent after SQL-queries of some sort,
    the server would crash.
  - if COM_REFRESH was requested with REFRESH_LOG only, and the error log
    failed to flush, the server would crash. The error log fails to flush
    when it points to unavailable file (for example, due to restricted
    permissions).

The fixes are:
  - call lex_start() in the beginning of COM_REFRESH;
  - handle failures to flush the error log properly, i.e. raise ER_UNKNOWN_ERROR.