Define INVALID_SOCKET and use it instead of SOCKET_BAD.

Revert the part of previous commit that defines SOCKET_BAD library wide.

src/agent.c

@@ -124,6 +124,10 @@ struct agent_ops {
     agent_disconnect_func disconnect;
 };
 
+#ifndef INVALID_SOCKET
+#define INVALID_SOCKET -1
+#endif
+
 struct _LIBSSH2_AGENT
 {
     LIBSSH2_SESSION *session;  /* the session this "belongs to" */
@@ -316,7 +320,7 @@ agent_transact_pageant(LIBSSH2_AGENT *agent, agent_transaction_ctx_t transctx)
 static int
 agent_disconnect_pageant(LIBSSH2_AGENT *agent)
 {
-    agent->fd = SOCKET_BAD;
+    agent->fd = INVALID_SOCKET;
     return 0;
 }
 
@@ -737,7 +741,7 @@ libssh2_agent_userauth(LIBSSH2_AGENT *agent,
 LIBSSH2_API int
 libssh2_agent_disconnect(LIBSSH2_AGENT *agent)
 {
-    if (agent->ops && agent->fd != SOCKET_BAD)
+    if (agent->ops && agent->fd != INVALID_SOCKET)
         return agent->ops->disconnect(agent);
     return 0;
 }
@@ -751,7 +755,7 @@ libssh2_agent_disconnect(LIBSSH2_AGENT *agent)
 LIBSSH2_API void
 libssh2_agent_free(LIBSSH2_AGENT *agent) {
     /* Allow connection freeing when the socket has lost its connection */
-    if (agent->fd != SOCKET_BAD) {
+    if (agent->fd != INVALID_SOCKET) {
         libssh2_agent_disconnect(agent);
     }
     agent_free_identities(agent);

src/libssh2_priv.h

@@ -151,10 +151,8 @@ static inline int writev(int sock, struct iovec *iov, int nvecs)
 
 #ifdef WIN32
 typedef SOCKET libssh2_socket_t;
-#define SOCKET_BAD ~0
 #else /* !WIN32 */
 typedef int libssh2_socket_t;
-#define SOCKET_BAD -1
 #endif /* WIN32 */
 
 /* RFC4253 section 6.1 Maximum Packet Length says: