Connection minor fixes

[zuiderkwast]

1. Remove redundant connIncrRefs/connDecrRefs

   In socket.c, the reference counter is incremented before calling callHandler, but the same reference counter is also incremented inside callHandler before calling the actual callback.

    static inline int callHandler(connection *conn, ConnectionCallbackFunc handler) {
        connIncrRefs(conn);
        if (handler) handler(conn);
        connDecrRefs(conn);
        ...
    }
   

   This commit removes the redundant incr/decr calls in socket.c

2. Correct return value of connRead for TLS when peer closed

   According to comments in connection.h, connRead returns 0 when the peer has closed the connection. This patch corrects the return value for TLS connections. (Without this patch, it returns -1 which means error.)

   There is an observable difference in what is logged in the verbose level: "Client closed connection" vs "Reading from client: (null)".

[zuiderkwast]

These minor fixes were written in January and already reviewed by @enjoy-binbin.

[enjoy-binbin]

LGTM.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.43%. Comparing base (04d76d8) to head (10fe21e).
Report is 1 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #953      +/-   ##
============================================
- Coverage     70.43%   70.43%   -0.01%     
============================================
  Files           113      113              
  Lines         61728    61725       -3     
============================================
- Hits          43479    43474       -5     
- Misses        18249    18251       +2     

Files	Coverage Δ
src/socket.c	91.12% <ø> (-0.16%)	⬇️
src/tls.c	100.00% <ø> (ø)

... and 11 files with indirect coverage changes