Fix String#slice! raise TypeError.

String#slice! raise TypeError:
```
% mruby -e '"0123456789".slice!(0, 11)'    
trace:
	[3] /home/tommy/work/mruby-iij/build/mrbgems/mruby-regexp-pcre/mrblib/string_pcre.rb:340:in String.splice
	[2] /home/tommy/work/mruby-iij/build/mrbgems/mruby-regexp-pcre/mrblib/string_pcre.rb:289:in String.[]=
	[1] /home/tommy/work/mruby-iij/build/mrbgems/mruby-regexp-pcre/mrblib/string_pcre.rb:186:in String.slice!
	[0] -e:1
/home/tommy/work/mruby-iij/build/mrbgems/mruby-regexp-pcre/mrblib/string_pcre.rb:340: expected String (TypeError)
```