terminal: fix out-of-bounds access in CSI W handler with no params

CSI ? W (cursor tabulation control) accessed input.params[0] without
first checking that params.len > 0, causing an index out-of-bounds
panic when the sequence had an intermediate but no parameters.

Add a params.len == 1 guard before accessing params[0].

Found by AFL++ fuzzing.

src/terminal/stream.zig

@@ -1188,7 +1188,10 @@ pub fn Stream(comptime Handler: type) type {
                         return;
                     },
 
-                    1 => if (input.intermediates[0] == '?' and input.params[0] == 5) {
+                    1 => if (input.intermediates[0] == '?' and
+                        input.params.len == 1 and
+                        input.params[0] == 5)
+                    {
                         try self.handler.vt(.tab_reset, {});
                     } else log.warn("invalid cursor tabulation control: {f}", .{input}),