assert makes incorrect assumption

[zoff99]

c-toxcore/toxcore/network.c

Line 1794 in 2824aa2

assert(count >= 0 && count <= UINT16_MAX);

count is often larger than UINT16_MAX, and then toxcore crashes.
i do not know if there is a bigger issue here with truncating "count" down to UINT16_MAX aswell.

this used to be "size_t":

c-toxcore/toxcore/network.c

Line 1468 in 4348b96

size_t net_socket_data_recv_buffer(Socket sock)

some refactoring was not correct after v0.2.13