patch 8.1.0702: ":sign place" only uses the current buffer

Problem:    ":sign place" only uses the current buffer.
Solution:   List signs for all buffers when there is no buffer argument.
            Fix error message for invalid buffer name in sign_place().
            (Yegappan Lakshmanan, closes #3774)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 58f5806..cd0888b 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -11462,7 +11462,7 @@
     buf = tv_get_buf(&argvars[3], FALSE);
     if (buf == NULL)
     {
-	EMSG2(_("E158: Invalid buffer name: %s"), tv_get_string(&argvars[2]));
+	EMSG2(_("E158: Invalid buffer name: %s"), tv_get_string(&argvars[3]));
 	goto cleanup;
     }