Absence of table in left part of LEFT/RIGHT join wasn't checked before
name resolution which resulted in NULL dereferencing and server crash.

Modified rules: 
"table_ref LEFT opt_outer JOIN_SYM table_ref" and "table_ref RIGHT opt_outer 
JOIN_SYM table_ref"
NULL check is moved before push_new_name_resolution_context()


sql/sql_yacc.yy:
  Fixed bug #15538: unchecked table absence caused server crash.
  Modified rules:
  "table_ref LEFT opt_outer JOIN_SYM table_ref" and "table_ref RIGHT opt_outer 
  JOIN_SYM table_ref"
  NULL check is moved before push_new_name_resolution_context()
mysql-test/r/select.result:
  Added test case for bug #15538: unchecked table absence caused server crash.
mysql-test/t/select.test:
  Added test case for bug #15538: unchecked table absence caused server crash.