vim-patch:9.0.1856: issues with formatting positional arguments (#25013)

Problem:  issues with formatting positional arguments
Solution: fix them, add tests and documentation

closes: vim/vim#12140
closes: vim/vim#12985

Tentatively fix message_test. Check NULL ptr.

aa90d4f031

Co-authored-by: Christ van Willegen <cvwillegen@gmail.com>

test/unit/strings_spec.lua

@@ -208,6 +208,7 @@ describe('vim_snprintf()', function()
       a('three one two', buf, bsize, '%3$s %1$s %2$s', 'one', 'two', 'three')
       a('1234567', buf, bsize, '%1$d', i(1234567))
       a('deadbeef', buf, bsize, '%1$x', u(0xdeadbeef))
+      a('001100', buf, bsize, '%2$0*1$b', i(6), u(12))
       a('001100', buf, bsize, '%1$0.*2$b', u(12), i(6))
       a('one two', buf, bsize, '%1$s %2$s', 'one', 'two')
       a('001100', buf, bsize, '%06b', u(12))