patch 8.0.0743: the 'termsize' option can be set to an invalid value

Problem:    The 'termsize' option can be set to an invalid value.
Solution:   Check the 'termsize' option to be valid.
diff --git a/src/testdir/gen_opt_test.vim b/src/testdir/gen_opt_test.vim
index 019c757..128a918 100644
--- a/src/testdir/gen_opt_test.vim
+++ b/src/testdir/gen_opt_test.vim
@@ -124,6 +124,7 @@
       \ 'switchbuf': [['', 'useopen', 'split,newtab'], ['xxx']],
       \ 'tagcase': [['smart', 'match'], ['', 'xxx', 'smart,match']],
       \ 'term': [[], []],
+      \ 'termsize': [['', '24x80', '0x80', '32x0', '0x0'], ['xxx', '80', '8ax9', '24x80b']],
       \ 'toolbar': [['', 'icons', 'text'], ['xxx']],
       \ 'toolbariconsize': [['', 'tiny', 'huge'], ['xxx']],
       \ 'ttymouse': [['', 'xterm'], ['xxx']],