patch 9.0.0351: message window may obscure the command line

Problem:    Message window may obscure the command line.
Solution:   Reduce the maximum height of the message window.
diff --git a/src/popupwin.c b/src/popupwin.c
index 72d38787..8688f3e 100644
--- a/src/popupwin.c
+++ b/src/popupwin.c
@@ -1356,6 +1356,8 @@
 
     if (wp->w_maxheight > 0)
 	maxheight = wp->w_maxheight;
+    else if (wp->w_popup_pos == POPPOS_BOTTOM)
+	maxheight = cmdline_row - 1;
 
     // start at the desired first line
     if (wp->w_firstline > 0)
@@ -4479,6 +4481,7 @@
 	message_win->w_popup_pos = POPPOS_BOTTOM;
 	message_win->w_wantcol = 1;
 	message_win->w_minwidth = 9999;
+	message_win->w_firstline = -1;
 
 	// no padding, border at the top
 	for (i = 0; i < 4; ++i)