buffer.h uses unsigned values for keeping track of allocated sizes.  This means that a call to nettle_buffer_grow could silently truncate a large argument of type size_t (on 64 bit systems), for example.

This issue also applies to nettle_realloc_func 

Furthermore, a size computation in nettle_buffer_grow can wrap around, failing to allocate additional buffer space:

  if (buffer->size + length > buffer->alloc)

There is another size computation in asn1_der_get_bignum which looks a bit suspicious as well.  If 8 * i->length wraps around, then the max_bits restriction is not necessarily enforced:

  if (max_bits && (8 * i->length > (16 + max_bits)))

This size computation in pgp_armor needs a guard against wraparound, too:

      unsigned text_size = BASE64_ENCODE_LENGTH(length)
	+ BASE64_ENCODE_FINAL_LENGTH;