socket: prevent recvfrom from returning garbage

In C recvfrom function sets addrlen parameter to zero when called on TCP
socket (at least on Linux). The src_addr parameter can contain garbage
in the case, so we should not dereference it.

Before this commit socket:recvfrom() can return 'from' table with only
family field (don't sure why, but addr->sa_family often contain PF_INET
value in my case) or return nil depending on the garbage at the address.
Now it always return nil.