The constructor of Lex_ident_sys returns LEX_CSTRING(NULL,0) if character set
conversion goes wrong, and raises the "wrong character string" error in
the diagnostics area.

The code in sql_yacc.yy did not check Lex_ident_sys::ptr against NULL,
so the execution entered functions that did not expect NULL (and crashed).

Fixing the code to do MYSQL_YYABORT if Lex_ident_sys::ptr is NULL
after constructing.