cuda: guard the iq4_nl dequantize row kernel against short rows (#29683)

dequantize_block_iq4_nl writes QK_K values per block, but a row can be shorter than that (an IQ4_NL row is only guaranteed to be a multiple of QK4_NL). Threads whose 32-value sub-block starts at or past k currently read and write past the end of the row. Skip those sub-blocks; for rows that are a multiple of QK_K the check never fires.