[Patch] Fix (similarly) libstdc++/9178 and 11305

[Paolo Carlini]

Pétur Runólfsson wrote:

>>+	      int __ext_multiplier = _M_codecvt->encoding();
>>+	      if (__ext_multiplier == -1 || __ext_multiplier == 0)
>>+		__ext_multiplier = _M_codecvt->max_length();
>>    
>>
>
>I think this can be simplified to
>
>    int __ext_multiplier = _M_codecvt->max_length();
>
>since _M_codecvt->encoding() == _M_codecvt->max_length() if
>_M_codecvt->encoding() > 0
>
Thanks...

>>+  virtual int
>>+  do_length(const StateT&, const extern_type* from,
>>+	    const extern_type* end, size_t max) const
>>+  {
>>    
>>
>According to the resolution to DR 75 (TC) the type of the first
>parameter of length() and do_length() is StateT&, not const StateT&
>(my fault, I wrote this testcase :-)
>
... and thanks!

Will commit with those changes.

Paolo.