Do not issue a 'read-only' error in case of DROP TEMPORARY TABLE on a non-existing temporary table.
Instead produce the correct "Unknown table" error or warning (in cases when the IF EXISTS clause was specified).

To a documentor: the part of the manual describing the 'read_only' system variable should be clarified to state the following:
"When the read_only variable is set to ON, all operations which create/update/drop tables are rejected with the exceptions for:
1. Any operation performed by the replication thread on a slave server
2. Any operation performed by a user that have the SUPER privilege
3. Any operation that creates/updates/drops only temporary tables"


mysql-test/r/read_only.result:
  Added testcases for bug #22077 "DROP TEMPORARY TABLE fails with wrong error if read_only is set"
mysql-test/t/read_only.test:
  Added testcases for bug #22077 "DROP TEMPORARY TABLE fails with wrong error if read_only is set"
sql/sql_parse.cc:
  Before issuing the 'read-only' error also check if the operation is not a DROP TEMPORARY TABLE statement