event/ServerSocket: add HAVE_UN check to AddAbstract()

NEWS

 ver 0.21.7 (not yet released)
 * require Meson 0.49.0 for native libgcrypt-config support
+* fix build failure with -Dlocal_socket=false
 
 ver 0.21.6 (2019/03/17)
 * protocol

src/event/ServerSocket.cxx

@@ -402,6 +402,11 @@ ServerSocket::AddPath(AllocatedPath &&path)
 void
 ServerSocket::AddAbstract(const char *name)
 {
+#if !defined(HAVE_UN)
+	(void)name;
+
+	throw std::runtime_error("Local socket support is disabled");
+#else
 	assert(name != nullptr);
 	assert(*name == '@');
 
@@ -409,6 +414,7 @@ ServerSocket::AddAbstract(const char *name)
 	address.SetLocal(name);
 
 	AddAddress(std::move(address));
+#endif
 }
 
 #endif