_mjit_compile_pc_and_sp.erb: make sure no uninitialized

area on VM stack to prevent SEGV on GC. GC may mark every value in VM
stack.

Unfortunately I couldn't write a test for it... So let me explain the
situation.

SEGV example:
https://gist.github.com/k0kubun/c7cea2b5761ffdff29ec79ea1a8f7f91

```
$ ruby --dump=insns -e 'def oct(num, len); "%0#{len}o" % num; end'
== disasm: #<ISeq:oct@-e:1 (1,0)-(1,41)> (catch: FALSE)
local table (size: 2, argc: 2 [opts: 0, rest: -1, post: 0, block: -1, kw: -1@-1, kwrest: -1])
[ 2] num@0<Arg> [ 1] len@1<Arg>
0000 putobject                    "%0"                                (   1)[LiCa]
0002 getlocal_WC_0                len@1
0004 dup
0005 checktype                    T_STRING
0007 branchif                     14
0009 dup
0010 opt_send_without_block       <callinfo!mid:to_s, argc:0, FCALL|ARGS_SIMPLE>, <callcache>
0013 tostring
0014 putobject                    "o"
0016 concatstrings                3
0018 getlocal_WC_0                num@0
0020 opt_mod                      <callinfo!mid:%, argc:1, ARGS_SIMPLE>, <callcache>
0023 leave                        [Re]
```

Prior to this commit, after arguments are pushed on 0010, stacks were:
VM stack: [uninitialized, uninitialized, len]
JIT stack: ["%0", len, len]

And then, when GC is invoked on 0016, VM stack will be [uninitialized, uninitialized]
and those uninitialized values will be marked by GC.

With this commit, after arguments are pushed on 0010, stacks will be:
VM stack: [len]
JIT stack: ["%0", len, len]

And VM stack will be [] on 0016.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63360 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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