patch 9.0.0311: test for hit-Enter prompt fails

Problem:    Test for hit-Enter prompt fails.
Solution:   Only reset cmdline_row when 'cmdheight' is zero.
diff --git a/src/drawscreen.c b/src/drawscreen.c
index aa5b8f7..ecbdd9f 100644
--- a/src/drawscreen.c
+++ b/src/drawscreen.c
@@ -831,8 +831,9 @@
     handle_any_postponed_drop();
 #endif
 
-    // in case it was changed in dont_use_message_window()
-    cmdline_row = Rows - p_ch;
+    if (p_ch == 0)
+	// in case it was changed in dont_use_message_window()
+	cmdline_row = Rows;
 }
 
 /*