TMP_ALLOC and size_t change

Hello, In master (unsigned int) was replaced with (size_t), that allows for even larger sizes to be input to encryption and decryption functions. However, the usage of TMP_ALLOC to make a copy of the input data (e.g., in ctr.c) contradicts that goal.

In general I think the usage of alloca() is dangerous as it not always known whether the stack is limited, e.g, when nettle is called in a co-routine or in application with non-growing stack. I think there can be an easy modification of TMP_ALLOC to use malloc for larger than 128 (or any other fixed number), and alloca otherwise. Would you be interested in such a patch? (it would require the introduction of a TMP_FREE as well)

regards, Nikos