PR libstdc++/87787 fix UBsan error in std::vector

	PR libstdc++/87787
	* include/bits/stl_uninitialized.h (__relocate_a_1): Do not call
	memmove when there's nothing to copy (and pointers could be null).

From-SVN: r265984

libstdc++-v3/ChangeLog

+2018-11-09  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/87787
+	* include/bits/stl_uninitialized.h (__relocate_a_1): Do not call
+	memmove when there's nothing to copy (and pointers could be null).
+
 2018-11-07  Hafiz Abid Qadeer  <abidh@codesourcery.com>
 
 	* configure: Regenerated.

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

@@ -904,7 +904,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 		   _Tp* __result, allocator<_Up>& __alloc)
     {
       ptrdiff_t __count = __last - __first;
-      __builtin_memmove(__result, __first, __count * sizeof(_Tp));
+      if (__count > 0)
+	__builtin_memmove(__result, __first, __count * sizeof(_Tp));
       return __result + __count;
     }