on SHOW CREATE TRIGGER + MERGE table

Problem: SHOW CREATE TRIGGER erroneously relies on fact
that we have the only underlying table for a trigger
(wrong for merge tables).

Fix: remove erroneous assert().


mysql-test/r/merge.result:
  Fix for bug #46614: Assertion in show_create_trigger() 
  on SHOW CREATE TRIGGER + MERGE table
    - test result.
mysql-test/t/merge.test:
  Fix for bug #46614: Assertion in show_create_trigger() 
  on SHOW CREATE TRIGGER + MERGE table
    - test case.
sql/sql_show.cc:
  Fix for bug #46614: Assertion in show_create_trigger() 
  on SHOW CREATE TRIGGER + MERGE table
    - unnecessary assert() removed as we may have more than 1 
  tables open e.g. for a merge table.