MATCH() function accepts column list as an argument. It was possible to override
this requirement with aliased non-column select expression. Which results in
server crash.

With this fix aliased non-column select expressions are not accepted by MATCH()
function, returning an error.

mysql-test/r/fulltext.result:
  A test case for BUG#36737.
mysql-test/t/fulltext.test:
  A test case for BUG#36737.
sql/item_func.cc:
  Only accept fields as arguments to MATCH().