REPAIR TABLE ... USE_FRM crashed debug servers.

A wrong assert assumed that this operation would not be executed
for MERGE tables.

Removed the assert.


mysql-test/r/merge.result:
  Bug#46339 - crash on REPAIR TABLE merge table USE_FRM
  Added test result.
mysql-test/t/merge.test:
  Bug#46339 - crash on REPAIR TABLE merge table USE_FRM
  Added test.
sql/sql_table.cc:
  Bug#46339 - crash on REPAIR TABLE merge table USE_FRM
  Removed false assert.