libstdc++: Micro-optimizaion for std::bitset stream extraction

Don't bother trying to copy any characters for bitset<0>.

libstdc++-v3/ChangeLog:

	* include/std/bitset (operator>>): Do not copy for N==0.
	* testsuite/20_util/bitset/io/input.cc: Add comment.

libstdc++-v3/include/std/bitset

@@ -1615,7 +1615,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 
       if (__tmp.empty() && _Nb)
 	__state |= __ios_base::failbit;
-      else
+      else if _GLIBCXX17_CONSTEXPR (_Nb)
 	__x._M_copy_from_string(__tmp, static_cast<size_t>(0), _Nb,
 				__zero, __one);
       if (__state)

libstdc++-v3/testsuite/20_util/bitset/io/input.cc

@@ -39,7 +39,7 @@ void test01()
   ss.clear();
   ss.str("*");
   ss >> b0;
-  VERIFY( ss.rdstate() == ios_base::goodbit );
+  VERIFY( ss.rdstate() == ios_base::goodbit ); // LWG 3199
 }
 
 int main()