BUG/MEDIUM: connection: don't store recv() result into trash.data

BUG/MEDIUM: connection: don't store recv() result into trash.data

Cyril Bonté discovered that the proxy protocol randomly fails since
commit 843b7cb ("MEDIUM: chunks: make the chunk struct's fields match
the buffer struct"). This is because we used to store recv()'s return
code into trash.data which is now unsigned, so it never compares as
negative against 0. Let's clean this up and test the result itself
without storing it first.

No backport is needed.