change behavior of mrb_sym2str

This patch affects some error messages.
This sort of message becomes
```
bin/mruby -e "send :'\0'"
trace:
	[0] -e:1
-e:1: undefined method '"\\0"' for main (NoMethodError)
```
like this:
```
bin/mruby -e "send :'\0'"
trace:
	[0] -e:1
-e:1: undefined method '\0' for main (NoMethodError)
```