Fix use-after-free for repeated fetchrow_arrayref calls when mysql_server_prepare=1

Function dbd_st_fetch() via Renew() can reallocate output buffer for
mysql_stmt_fetch() call. But it does not update pointer to that buffer in
imp_sth->stmt structure initialized by mysql_stmt_bind_result() function.
That leads to use-after-free in any mysql function which access
imp_sth->stmt structure (e.g. mysql_stmt_fetch()).

This patch fix this problem and properly updates pointer in imp_sth->stmt
structure after Renew() call.

Test 40server_prepare_crash.t is extended to check for that use-after-free
crash.