Trailing comma in expression crashes Tree.dump

[ehaas]

Found using fuzz testing

void foo(void) {
    1,;
}

Compile with --verbose-ast:

thread 14001920 panic: reached unreachable code
/Users/ehaas/source/zig/build/lib/zig/std/debug.zig:226:14: 0x10a4dc2f8 in std.debug.assert (arocc)
    if (!ok) unreachable; // assertion failure
             ^
/Users/ehaas/source/arocc/src/Tree.zig:557:21: 0x10a52235b in Tree.dumpNode (arocc)
    std.debug.assert(node != .none);
                    ^
/Users/ehaas/source/arocc/src/Tree.zig:905:30: 0x10a52a0c9 in Tree.dumpNode (arocc)
            try tree.dumpNode(data.bin.rhs, level + delta, w);
                             ^
/Users/ehaas/source/arocc/src/Tree.zig:688:57: 0x10a528297 in Tree.dumpNode (arocc)
            if (data.bin.lhs != .none) try tree.dumpNode(data.bin.lhs, level + delta, w);
                                                        ^
/Users/ehaas/source/arocc/src/Tree.zig:628:30: 0x10a527bf9 in Tree.dumpNode (arocc)
            try tree.dumpNode(data.decl.node, level + delta, w);
                             ^
/Users/ehaas/source/arocc/src/Tree.zig:541:26: 0x10a50ae64 in Tree.dump (arocc)
        try tree.dumpNode(i, 0, writer);
                         ^
/Users/ehaas/source/arocc/src/main.zig:262:18: 0x10a4ecc2d in processSource (arocc)
        tree.dump(buf_writer.writer()) catch {};
                 ^
/Users/ehaas/source/arocc/src/main.zig:223:22: 0x10a4e1161 in handleArgs (arocc)
        processSource(comp, source, builtin, user_macros) catch |e| switch (e) {
                     ^
/Users/ehaas/source/arocc/src/main.zig:41:15: 0x10a4de2ed in main (arocc)
    handleArgs(&comp, args) catch |err| switch (err) {
              ^
/Users/ehaas/source/zig/build/lib/zig/std/start.zig:524:29: 0x10a55bd7c in std.start.callMain (arocc)
            return root.main();
                            ^
/Users/ehaas/source/zig/build/lib/zig/std/start.zig:469:12: 0x10a4e2267 in std.start.callMainWithArgs (arocc)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/Users/ehaas/source/zig/build/lib/zig/std/start.zig:434:12: 0x10a4e21a5 in std.start.main (arocc)
    return @call(.{ .modifier = .always_inline }, callMainWithArgs, .{ @intCast(usize, c_argc), c_argv, envp });