patch 9.0.0191: messages test fails; window size incorrect

Problem:    Messages test fails; window size incorrect when 'cmdheight' is
            made smaller.
Solution:   Properly cleanup after test with cmdheight zero.  Resize windows
            correctly when 'cmdheight' gets smaller.
diff --git a/src/window.c b/src/window.c
index 5b192cc..7d2fe42 100644
--- a/src/window.c
+++ b/src/window.c
@@ -6555,6 +6555,12 @@
     if (p_ch > old_p_ch && cmdline_row <= Rows - p_ch)
 	return;
 
+    // If cmdline_row is smaller than what it is supposed to be for 'cmdheight'
+    // then set old_p_ch to what it would be, so that the windows get resized
+    // properly for the new value.
+    if (cmdline_row < Rows - p_ch)
+	old_p_ch = Rows - cmdline_row;
+
     // Find bottom frame with width of screen.
     frp = lastwin->w_frame;
     while (frp->fr_width != Columns && frp->fr_parent != NULL)