static_string<0> does not memcpy

fix #41

include/boost/static_string/static_string.hpp

@@ -1319,7 +1319,7 @@ public:
   basic_static_string&
   assign(const basic_static_string& s) noexcept
   {
-    if (this == &s)
+    if (data() == s.data())
       return *this;
     return assign_unchecked(s.data(), s.size());
   }

test/static_string.cpp

@@ -674,6 +674,13 @@ testAssignment()
 
     s_long.assign(s_long.data() + 2, 8);
     BOOST_TEST(s_long == "rem ipsu");
+
+    // issue #41
+    {
+        boost::static_strings::static_string<0> a;
+        auto b = a;
+        BOOST_TEST(b == "");
+    }
 }
 
 // done