patch 8.1.2281: 'showbreak' cannot be set for one window

Problem:    'showbreak' cannot be set for one window.
Solution:   Make 'showbreak' global-local.
diff --git a/src/optionstr.c b/src/optionstr.c
index bcc737e..143c8dd 100644
--- a/src/optionstr.c
+++ b/src/optionstr.c
@@ -1440,12 +1440,12 @@
 
 #ifdef FEAT_LINEBREAK
     // 'showbreak'
-    else if (varp == &p_sbr)
+    else if (gvarp == &p_sbr)
     {
-	for (s = p_sbr; *s; )
+	for (s = *varp; *s; )
 	{
 	    if (ptr2cells(s) != 1)
-		errmsg = N_("E595: contains unprintable or wide character");
+		errmsg = N_("E595: 'showbreak' contains unprintable or wide character");
 	    MB_PTR_ADV(s);
 	}
     }