patch 9.0.0340: the 'cmdheight' zero support causes too much trouble

Problem:    The 'cmdheight' zero support causes too much trouble.
Solution:   Revert support for 'cmdheight' being zero.
diff --git a/src/testdir/gen_opt_test.vim b/src/testdir/gen_opt_test.vim
index a552160..83f43f7 100644
--- a/src/testdir/gen_opt_test.vim
+++ b/src/testdir/gen_opt_test.vim
@@ -27,7 +27,7 @@
 " Two lists with values: values that work and values that fail.
 " When not listed, "othernum" or "otherstring" is used.
 let test_values = {
-      \ 'cmdheight': [[0, 1, 2, 10], [-1]],
+      \ 'cmdheight': [[1, 2, 10], [-1, 0]],
       \ 'cmdwinheight': [[1, 2, 10], [-1, 0]],
       \ 'columns': [[12, 80], [-1, 0, 10]],
       \ 'conceallevel': [[0, 1, 2, 3], [-1, 4, 99]],