patch 9.0.1078: with the +vartabs feature indent folding may use wrong 'ts'

Problem:    With the +vartabs feature indent folding may use wrong 'tabstop'.
Solution:   Use the "buf" argument instead of "curbuf".
diff --git a/src/indent.c b/src/indent.c
index aaf3caa..cbb3f94 100644
--- a/src/indent.c
+++ b/src/indent.c
@@ -420,7 +420,7 @@
 {
 # ifdef FEAT_VARTABS
     return get_indent_str_vtab(ml_get_buf(buf, lnum, FALSE),
-			       (int)curbuf->b_p_ts, buf->b_p_vts_array, FALSE);
+			       (int)buf->b_p_ts, buf->b_p_vts_array, FALSE);
 # else
     return get_indent_str(ml_get_buf(buf, lnum, FALSE), (int)buf->b_p_ts, FALSE);
 # endif