eval: Fix QuickBuild failures

Compiler used by one VM in QuickBuild has found a number of false positives.
Everything is fine on travis.

List of failures:

From [QuickBuild][1], build [7429][2]:

    14:38:19,945 WARN  - /home/quickbuild/buildagent/workspace/root/neovim/pull-requests-automated/src/nvim/eval.c: In function ‘assert_bool’:
    14:38:19,945 WARN  - /home/quickbuild/buildagent/workspace/root/neovim/pull-requests-automated/src/nvim/eval.c:7551:40: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
    14:38:20,058 WARN  - cc1: all warnings being treated as errors

. This is not making much sense (7551:40 is `!=` in `{SpecialVarValue} !=
({bool}?{SpecialVarValue}:{SpecialVarValue})`), but this error is present.

---

Also fail from [build][3] [4930][4]:

    15:47:00,853 WARN  - /home/quickbuild/buildagent/workspace/root/neovim/pull-requests-automated/src/nvim/eval/encode.c: In function ‘encode_read_from_list’:
    15:47:00,853 WARN  - /home/quickbuild/buildagent/workspace/root/neovim/pull-requests-automated/src/nvim/eval/encode.c:258:30: error: conversion to ‘char’ from ‘int’ may alter its value [-Werror=conversion]

, pointing to `:` in `{char} = ({char} == {const} ? {const} : {char})` where
`{const}` is character constant like `'\n'`. I have no idea where exactly it saw
conversion, so simply casted everything to (char).

---

[Build][5] error:

    08:32:03,472 WARN  - /home/quickbuild/buildagent/workspace/root/neovim/pull-requests-automated/src/nvim/eval.c: In function ‘tv_equal’:
    08:32:03,472 WARN  - /home/quickbuild/buildagent/workspace/root/neovim/pull-requests-automated/src/nvim/eval.c:5077:1: error: control reaches end of non-void function [-Werror=return-type]

---

Build [4949][7]:

    11:28:00,578 WARN  - /home/quickbuild/buildagent/workspace/root/neovim/pull-requests-automated/src/nvim/eval.c: In function ‘f_type’:
    11:28:00,578 WARN  - /home/quickbuild/buildagent/workspace/root/neovim/pull-requests-automated/src/nvim/eval.c:16085:24: error: ‘n’ may be used uninitialized in this function [-Werror=uninitialized]
    11:28:00,581 WARN  - /home/quickbuild/buildagent/workspace/root/neovim/pull-requests-automated/src/nvim/eval.c: In function ‘f_empty’:
    11:28:00,581 WARN  - /home/quickbuild/buildagent/workspace/root/neovim/pull-requests-automated/src/nvim/eval.c:8505:24: error: ‘n’ may be used uninitialized in this function [-Werror=uninitialized]

[1]: http://neovim-qb.szakmeister.net/wicket/page?5-1.ILinkListener-content-buildTab-panel-masterStep-body-children-0-step-body-children-2-body-children-3-step-body-children-0-step-body-children-0-step-head-logLink
[2]: http://neovim-qb.szakmeister.net/build/4929
[3]: http://neovim-qb.szakmeister.net/build/4930
[4]: http://neovim-qb.szakmeister.net/wicket/page?1-1.ILinkListener-content-buildTab-panel-masterStep-body-children-0-step-body-children-1-body-children-3-step-body-children-0-step-body-children-0-step-head-logLink
[5]: http://neovim-qb.szakmeister.net/build/4948/step_status
[7]: http://neovim-qb.szakmeister.net/build/4949

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