net.c: always initialize bytes_read
net.c: always initialize bytes_read

In CID 1459612, Coverity scan has complained about uninitialized use
of the variable bytes_read. Although this is a false positive here,
it seems to be more safe to always initialize bytes_read.

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap