Fix itsNextName not clearing when not found

An issue exists when loading vectors of objects where, if the last nvp of
the previous object does not exist in the json file, the itsNextName
variable within the json serializer is not cleared. This causes the vector
serializer to search for that name next (when it should be searching for a
nameless object.) The json serializer then throws during the named search.

Mild reworking of itsNextName solution

include/cereal/archives/json.hpp

@@ -564,18 +564,20 @@ namespace cereal
           @throws Exception if an expectedName is given and not found */
       inline void search()
       {
+        // store pointer to itsNextName locally and reset to nullptr in case search() throws
+        auto localNextName = itsNextName;
+        itsNextName = nullptr;
+
         // The name an NVP provided with setNextName()
-        if( itsNextName )
+        if( localNextName )
         {
           // The actual name of the current node
           auto const actualName = itsIteratorStack.back().name();
 
           // Do a search if we don't see a name coming up, or if the names don't match
-          if( !actualName || std::strcmp( itsNextName, actualName ) != 0 )
-            itsIteratorStack.back().search( itsNextName );
+          if( !actualName || std::strcmp( localNextName, actualName ) != 0 )
+            itsIteratorStack.back().search( localNextName );
         }
-
-        itsNextName = nullptr;
       }
 
     public: