A static analysis tool suggested that in the function
row_merge_read_clustered_index(), ut_free(nonnull) could
be invoked twice for nonnull!=NULL. While a manual review
of the code disproved this, it should not hurt to clean up
the code so that the static analysis tool will not complain.

index_tuple_info_t::insert(), row_mtuple_cmp(): Remove the
parameter mtr_committed, which duplicated !mtr->is_active().

row_merge_read_clustered_index(): Initialize row_heap = NULL.
Remove a duplicated call mem_heap_empty(row_heap) that was
inadvertently added in commit cb1e76e4.

Replace a "goto func_exit" with "break", to get consistent error
handling for both failures to create or write a temporary file.

end_of_index: Assign row_heap=NULL and nonnull=NULL to prevent
double freeing.

func_exit: Check for row_heap!=NULL before invoking mem_heap_free().

Closes #959