short circuit serializer::read on empty buffer

Author: grisumbras

include/boost/json/impl/serializer.ipp

@@ -542,6 +542,9 @@ read(char* dest, std::size_t size)
     if( !fn0_ )
         reset(nullptr);
 
+    if(BOOST_JSON_UNLIKELY(size == 0))
+        return {dest, 0};
+
     detail::stream ss(dest, size);
     if(st_.empty())
         fn0_(*this, ss);

test/serializer.cpp

@@ -896,6 +896,10 @@ class serializer_test
     void
     run()
     {
+        {
+            serializer sr;
+            BOOST_ASSERT( sr.read(nullptr, 0).empty() );
+        }
         testNull();
         testBoolean();
         testString();