patch 8.2.2933: when 'clipboard' is "unnamed" zp does not work correctly

Problem:    When 'clipboard' is "unnamed" zp and zP do not work correctly.
Solution:   Pass -1 to str_to_reg() and fix computing the character width
            instead of using the byte length. (Christian Brabandt,
            closes #8301, closes #8317)
diff --git a/src/clipboard.c b/src/clipboard.c
index c263492..b1da99b 100644
--- a/src/clipboard.c
+++ b/src/clipboard.c
@@ -2090,7 +2090,7 @@
 
     clip_free_selection(cbd);
 
-    str_to_reg(y_ptr, type, str, len, 0L, FALSE);
+    str_to_reg(y_ptr, type, str, len, -1, FALSE);
 }
 
 /*