Add type check in Hash#merge!

Hash.merge! doesn't nil check.
```
bin/mruby -e "{}.merge!(nil)"
trace:
	[0] -e:1
-e:1: undefined method 'each_key' for nil (NoMethodError)
```

After fix:
```
bin/mruby -e "{}.merge!(nil)"                               
trace:
	[0] -e:1
-e:1: can't convert argument into Hash (RuntimeError)
```