MDEV-18873 Server crashes in Compare_identifiers::operator or in my_strcasecmp_utf8 upon ADD PERIOD IF NOT EXISTS with empty name

empty identifier specified as `` ends up with a NULL LEX_CSTRING::str in lexer.
This is not considered correct in upper layers, for example in Compare_identifiers::operator().
Empty column name is usually avoided by a check_column_name() call while parsing,
and period name matches the column name completely.
Hence, this fix uses the mentioned call for verification, too.

1 files changed, 1 insertions, 0 deletions

diff --git a/sql/table.h b/sql/table.h
index c9503b54934..3f750e7b221 100644
--- a/sql/table.h
+++ b/sql/table.h
@@ -3231,6 +3231,7 @@ void open_table_error(TABLE_SHARE *share, enum open_frm_error error,
 void update_create_info_from_table(HA_CREATE_INFO *info, TABLE *form);
 bool check_db_name(LEX_STRING *db);
 bool check_column_name(const char *name);
+bool check_period_name(const char *name);
 bool check_table_name(const char *name, size_t length, bool check_for_path_chars);
 int rename_file_ext(const char * from,const char * to,const char * ext);
 char *get_field(MEM_ROOT *mem, Field *field);