patch 8.0.0681: unnamed register only contains the last deleted text

Problem:    Unnamed register only contains the last deleted text when
            appending deleted text to a register. (Wolfgang Jeltsch)
Solution:   Only set y_previous when not using y_append. (Christian Brabandt)
diff --git a/src/ops.c b/src/ops.c
index 7cbc0c2..2aec5d4 100644
--- a/src/ops.c
+++ b/src/ops.c
@@ -1636,7 +1636,9 @@
     free_yank_all();			/* free register nine */
     for (n = 9; n > 1; --n)
 	y_regs[n] = y_regs[n - 1];
-    y_previous = y_current = &y_regs[1];
+    y_current = &y_regs[1];
+    if (!y_append)
+	y_previous = y_current;
     y_regs[1].y_array = NULL;		/* set register one to empty */
 }