posix: sockets: address was not set in recvfrom

The address is an optional parameter of recvfrom. If it is
not null, recvfrom must store the address of the sender.
However this was only allowed if res was equal to 0, which
is wrong since res contains the number of bytes received
or -1.
This commit ensures that the address is set only if no
previous errors happened before.

Signed-off-by: Francois Berder <francois.berder@imgtec.com>