Set errno in CListener::Listen()

CSocket doesn't necessarily set errno, but the callers of Listen() assume an
error message in errno.

Signed-off-by: Uli Schlachter <psychon@znc.in>

Listener.cpp

@@ -16,6 +16,7 @@ CListener::~CListener() {
 
 bool CListener::Listen() {
 	if (!m_uPort || m_pListener) {
+		errno = EINVAL;
 		return false;
 	}
 
@@ -29,6 +30,10 @@ bool CListener::Listen() {
 	}
 #endif
 
+	// If e.g. getaddrinfo() fails, the following might not set errno.
+	// Make sure there is a consistent error message, not something random
+	// which might even be "Error: Success".
+	errno = EINVAL;
 	return CZNC::Get().GetManager().ListenHost(m_uPort, "_LISTENER", m_sBindHost, bSSL, SOMAXCONN,
 			m_pListener, 0, m_eAddr);
 }