PR libstdc++/85843 fix "should be explicitly initialized" warnings

	PR libstdc++/85843
	* src/c++11/cow-stdexcept.cc (logic_error, runtime_error): Explicitly
	initialize base class to avoid warnings.

From-SVN: r260427

libstdc++-v3/ChangeLog

+2018-05-21  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/85843
+	* src/c++11/cow-stdexcept.cc (logic_error, runtime_error): Explicitly
+	initialize base class to avoid warnings.
+
 2018-05-19  Jonathan Wakely  <jwakely@redhat.com>
 
 	* src/c++11/codecvt.cc (__codecvt_utf8_base<wchar_t>::do_in)

libstdc++-v3/src/c++11/cow-stdexcept.cc

@@ -56,13 +56,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // Copy constructors and assignment operators defined using COW std::string
 
   logic_error::logic_error(const logic_error& e) noexcept
-  : _M_msg(e._M_msg) { }
+  : exception(e), _M_msg(e._M_msg) { }
 
   logic_error& logic_error::operator=(const logic_error& e) noexcept
   { _M_msg = e._M_msg; return *this; }
 
   runtime_error::runtime_error(const runtime_error& e) noexcept
-  : _M_msg(e._M_msg) { }
+  : exception(e), _M_msg(e._M_msg) { }
 
   runtime_error&
   runtime_error::operator=(const runtime_error& e) noexcept