HANDLER OPEN

The problem was a too restrictive assert in the code for 
HANDLER ... OPEN and HANDLER ... READ that checked table->next
to verify that we didn't open views or merge tables.

This pointer is also used to link temporary tables together
(see thd->temporary_tables). In this case TABLE::next can be
set even if we're trying to open a single table.

This patch adjust the two asserts to also check for the presence
of temporary tables.

Test case added to handler_myisam.test.