Avoid -Woverflow warning in __numeric_limits_integer

This is the same fix as was done for std::numeric_limits in r183905.

	PR libstdc++/52119
	* include/ext/numeric_traits.h (__glibcxx_min): Avoid integer
	overflow warning with -Wpedantic -Wsystem-headers.

From-SVN: r270858

libstdc++-v3/ChangeLog

+2019-05-03  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/52119
+	* include/ext/numeric_traits.h (__glibcxx_min): Avoid integer
+	overflow warning with -Wpedantic -Wsystem-headers.
+
 2019-05-02  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR libstdc++/90314

libstdc++-v3/include/ext/numeric_traits.h

@@ -39,13 +39,13 @@ namespace __gnu_cxx _GLIBCXX_VISIBILITY(default)
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   // Compile time constants for builtin types.
-  // Sadly std::numeric_limits member functions cannot be used for this.
+  // In C++98 std::numeric_limits member functions cannot be used for this.
 #define __glibcxx_signed(_Tp) ((_Tp)(-1) < 0)
 #define __glibcxx_digits(_Tp) \
   (sizeof(_Tp) * __CHAR_BIT__ - __glibcxx_signed(_Tp))
 
 #define __glibcxx_min(_Tp) \
-  (__glibcxx_signed(_Tp) ? (_Tp)1 << __glibcxx_digits(_Tp) : (_Tp)0)
+  (__glibcxx_signed(_Tp) ? -__glibcxx_max(_Tp) - 1 : (_Tp)0)
 
 #define __glibcxx_max(_Tp) \
   (__glibcxx_signed(_Tp) ? \