udf: remove unnecessary test in udf_build_ustr_exact()

We can remove parameter checks:

udf_build_ustr_exact() is only called by udf_get_filename()
which now assures dest is not NULL

udf_find_entry() and udf_readdir() call udf_get_filename()
after checking sname
udf_symlink_filler() calls udf_pc_to_char() with sname=kmap(page).

udf_find_entry() and udf_readdir() call udf_get_filename with UDF_NAME_LEN
udf_pc_to_char() with PAGE_SIZE

Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Jan Kara <jack@suse.cz>