patch 8.2.1947: crash when using "zj" without folds

Problem:    Crash when using "zj" without folds. (Sean Dewar)
Solution:   Check for at least one fold. (closes #7245)
diff --git a/src/fold.c b/src/fold.c
index 8a904f6..4b4e618 100644
--- a/src/fold.c
+++ b/src/fold.c
@@ -902,6 +902,8 @@
 	// that moves the cursor is used.
 	lnum_off = 0;
 	gap = &curwin->w_folds;
+	if (gap->ga_len == 0)
+	    break;
 	use_level = FALSE;
 	maybe_small = FALSE;
 	lnum_found = curwin->w_cursor.lnum;