PR libstdc++/81173 fix undefined memset with null pointer

	PR libstdc++/81173
	* include/bits/stl_bvector.h (vector<bool>::_M_initialize_value):
	Do not pass null pointer to memset.

From-SVN: r249554

libstdc++-v3/ChangeLog

+2017-06-22  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/81173
+	* include/bits/stl_bvector.h (vector<bool>::_M_initialize_value):
+	Do not pass null pointer to memset.
+
 2017-06-21  Ville Voutilainen  <ville.voutilainen@gmail.com>
 
 	PR libstdc++/80675

libstdc++-v3/include/bits/stl_bvector.h

@@ -406,7 +406,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
   {
     if (__first._M_p != __last._M_p)
       {
-	_Bit_type *__first_p = __first._M_p;
+	_Bit_type* __first_p = __first._M_p;
 	if (__first._M_offset != 0)
 	  __fill_bvector(__first_p++, __first._M_offset, _S_word_bit, __x);
 
@@ -1129,8 +1129,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
       void
       _M_initialize_value(bool __x)
       {
-	__builtin_memset(this->_M_impl._M_start._M_p, __x ? ~0 : 0,
-		(this->_M_impl._M_end_addr() - this->_M_impl._M_start._M_p)
+	if (_Bit_type* __p = this->_M_impl._M_start._M_p)
+	  __builtin_memset(__p, __x ? ~0 : 0,
+			   (this->_M_impl._M_end_addr() - __p)
 			   * sizeof(_Bit_type));
       }