fix: guard ioctl results with when to avoid partial values

ignore-errors + ioctl can return (values 80 nil) when the fd exists
but isn't a terminal. or propagates partial values, causing nil in
w or h. Wrap with multiple-value-bind + when to filter.

src/backend/simple.lisp

@@ -37,9 +37,14 @@
                     (values (sb-alien:deref winsize 1)
                             (sb-alien:deref winsize 0)))
                (sb-alien:free-alien winsize)))))
-    (or (ignore-errors (ioctl-size 0))
-        (ignore-errors
-          (ioctl-size (sb-sys:fd-stream-fd (backend-output-stream b))))
+    (or (multiple-value-bind (cols rows) (ignore-errors (ioctl-size 0))
+          (when (and cols rows (> cols 0) (> rows 0))
+            (values cols rows)))
+        (multiple-value-bind (cols rows)
+            (ignore-errors
+              (ioctl-size (sb-sys:fd-stream-fd (backend-output-stream b))))
+          (when (and cols rows (> cols 0) (> rows 0))
+            (values cols rows)))
         (ignore-errors
           (let* ((cstr (sb-ext:posix-getenv "COLUMNS"))
                  (rstr (sb-ext:posix-getenv "LINES"))