suppress the warnings from get_valid_iv_sym()

```
$ make
ruby ./minirake
(in /home/liyazhou/code/mruby)
CC    src/kernel.c -> build/host/src/kernel.o
/home/liyazhou/code/mruby/src/kernel.c: In function ‘mrb_obj_ivar_get’:
/home/liyazhou/code/mruby/src/kernel.c:572: warning: ‘iv_name_id’ may be used uninitialized in this function
/home/liyazhou/code/mruby/src/kernel.c:572: note: ‘iv_name_id’ was declared here
/home/liyazhou/code/mruby/src/kernel.c: In function ‘mrb_obj_ivar_defined’:
/home/liyazhou/code/mruby/src/kernel.c:572: warning: ‘iv_name_id’ may be used uninitialized in this function
/home/liyazhou/code/mruby/src/kernel.c:572: note: ‘iv_name_id’ was declared here
/home/liyazhou/code/mruby/src/kernel.c: In function ‘mrb_obj_ivar_set’:
/home/liyazhou/code/mruby/src/kernel.c:572: warning: ‘iv_name_id’ may be used uninitialized in this function
/home/liyazhou/code/mruby/src/kernel.c:572: note: ‘iv_name_id’ was declared here
```

btw, The parameter iv_name SHOULD be string or symbol, but the current `if () else if ()... ` code make no garantee on it.