Fix crash in mnesia_controller due to a function_clause exception from is_tab_blocked/1

If mnesia_controller:block_table/1 is called twice for same table,
the new stored value of {Tab, where_to_commit} will be {blocked, {blocked,List}}.
This will result in an exception when calling is_tab_blocked/1.
mnesia_conttoller:block_table/1 should check if the table is already blocked.
Merge branch 'dg/win_icon' into pu
Merge branch 'rc/break-out-asserts' into pu
Merge branch 'nm/mnesia_idx_insert_speedup' into pu
Merge branch 'tja/erl_pp_options' into pu
Merge branch 'nox/disambiguate-equal-binary' into pu
Merge branch 'hsv/add_init_to_lists' into pu
Merge branch 'nox/fix-epp-file-attrs' into pu
Merge branch 'nox/fix-silent-rules' into pu
Merge branch 'nox/fix-warnings' into pu
Merge branch 'nox/wide-chars' into pu
Merge branch 'sla/reltool_copy_file_not_owner_fix' into pu
Merge branch 'sla/odbc_in_non_standard_location' into pu
Merge branch 'avi/fix-os-cmd-win' into pu
Merge branch 'ao/fix_mnesia_block_table_exception' into pu
Merge branch 'bjorn/fix-encoding' into pu