patch 9.1.0808: Terminal scrollback doesn't shrink when decreasing 'termwinscroll'

Problem:  Terminal scrollback doesn't shrink when reducing
          'termwinscroll'
Solution: Check if option value was decreased (Milly).

closes: #15904

Signed-off-by: Milly <milly.ca@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/option.c

@@ -4259,6 +4259,26 @@ did_set_termguicolors(optset_T *args UNUSED)
 }
 #endif
 
+#if defined(FEAT_TERMINAL) || defined(PROTO)
+/*
+ * Process the updated 'termwinscroll' option value.
+ */
+    char *
+did_set_termwinscroll(optset_T *args)
+{
+    long	*pp = (long *)args->os_varp;
+    char	*errmsg = NULL;
+
+    if (*pp < 1)
+    {
+	errmsg = e_argument_must_be_positive;
+	*pp = 1;
+    }
+
+    return errmsg;
+}
+#endif
+
 /*
  * Process the updated 'terse' option value.
  */