eval/encode: Fix writing strings starting with NL to list

Error [found][1] by oni-link.

[1]: https://github.com/neovim/neovim/pull/4131/files#r52239384

test/unit/eval/encode_spec.lua

@@ -0,0 +1,100 @@
+local helpers = require('test.unit.helpers')
+local eval_helpers = require('test.unit.eval.helpers')
+
+local cimport = helpers.cimport
+local to_cstr = helpers.to_cstr
+local eq = helpers.eq
+
+local list = eval_helpers.list
+local lst2tbl = eval_helpers.lst2tbl
+local type_key = eval_helpers.type_key
+local list_type = eval_helpers.list_type
+local null_string = eval_helpers.null_string
+
+local encode = cimport('./src/nvim/eval/encode.h')
+
+describe('encode_list_write()', function()
+  local encode_list_write = function(l, s)
+    return encode.encode_list_write(l, to_cstr(s), #s)
+  end
+
+  it('writes empty string', function()
+    local l = list()
+    eq(0, encode_list_write(l, ''))
+    eq({[type_key]=list_type}, lst2tbl(l))
+  end)
+
+  it('writes ASCII string literal with printable characters', function()
+    local l = list()
+    eq(0, encode_list_write(l, 'abc'))
+    eq({[type_key]=list_type, 'abc'}, lst2tbl(l))
+  end)
+
+  it('writes string starting with NL', function()
+    local l = list()
+    eq(0, encode_list_write(l, '\nabc'))
+    eq({[type_key]=list_type, null_string, 'abc'}, lst2tbl(l))
+  end)
+
+  it('writes string starting with NL twice', function()
+    local l = list()
+    eq(0, encode_list_write(l, '\nabc'))
+    eq({[type_key]=list_type, null_string, 'abc'}, lst2tbl(l))
+    eq(0, encode_list_write(l, '\nabc'))
+    eq({[type_key]=list_type, null_string, 'abc', 'abc'}, lst2tbl(l))
+  end)
+
+  it('writes string ending with NL', function()
+    local l = list()
+    eq(0, encode_list_write(l, 'abc\n'))
+    eq({[type_key]=list_type, 'abc', null_string}, lst2tbl(l))
+  end)
+
+  it('writes string ending with NL twice', function()
+    local l = list()
+    eq(0, encode_list_write(l, 'abc\n'))
+    eq({[type_key]=list_type, 'abc', null_string}, lst2tbl(l))
+    eq(0, encode_list_write(l, 'abc\n'))
+    eq({[type_key]=list_type, 'abc', 'abc', null_string}, lst2tbl(l))
+  end)
+
+  it('writes string starting, ending and containing NL twice', function()
+    local l = list()
+    eq(0, encode_list_write(l, '\na\nb\n'))
+    eq({[type_key]=list_type, null_string, 'a', 'b', null_string}, lst2tbl(l))
+    eq(0, encode_list_write(l, '\na\nb\n'))
+    eq({[type_key]=list_type, null_string, 'a', 'b', null_string, 'a', 'b', null_string}, lst2tbl(l))
+  end)
+
+  it('writes string starting, ending and containing NUL with NL between twice', function()
+    local l = list()
+    eq(0, encode_list_write(l, '\0\n\0\n\0'))
+    eq({[type_key]=list_type, '\n', '\n', '\n'}, lst2tbl(l))
+    eq(0, encode_list_write(l, '\0\n\0\n\0'))
+    eq({[type_key]=list_type, '\n', '\n', '\n\n', '\n', '\n'}, lst2tbl(l))
+  end)
+
+  it('writes string starting, ending and containing NL with NUL between twice', function()
+    local l = list()
+    eq(0, encode_list_write(l, '\n\0\n\0\n'))
+    eq({[type_key]=list_type, null_string, '\n', '\n', null_string}, lst2tbl(l))
+    eq(0, encode_list_write(l, '\n\0\n\0\n'))
+    eq({[type_key]=list_type, null_string, '\n', '\n', null_string, '\n', '\n', null_string}, lst2tbl(l))
+  end)
+
+  it('writes string containing a single NL twice', function()
+    local l = list()
+    eq(0, encode_list_write(l, '\n'))
+    eq({[type_key]=list_type, null_string, null_string}, lst2tbl(l))
+    eq(0, encode_list_write(l, '\n'))
+    eq({[type_key]=list_type, null_string, null_string, null_string}, lst2tbl(l))
+  end)
+
+  it('writes string containing a few NLs twice', function()
+    local l = list()
+    eq(0, encode_list_write(l, '\n\n\n'))
+    eq({[type_key]=list_type, null_string, null_string, null_string, null_string}, lst2tbl(l))
+    eq(0, encode_list_write(l, '\n\n\n'))
+    eq({[type_key]=list_type, null_string, null_string, null_string, null_string, null_string, null_string, null_string}, lst2tbl(l))
+  end)
+end)