Zero-length variables caused failures when using the length to look
up the name in a hash.  Instead, signal that no zero-length name can
ever be found and that to encounter one is a syntax error.


mysql-test/r/variables.result:
  Results for test.
mysql-test/t/variables.test:
  Insert tests to prove that zero-length variable names do not cause
  faults.
sql/gen_lex_hash.cc:
  If the length is zero, then there is nothing to look-up in the 
  hash.
sql/sql_lex.cc:
  Names of variables must not be empty.  Signal an error of that 
  happens.