fix memory leak in search engine

while performing case insensitive and exact match searching Song was
really copied, so clearing its internal mpd_Song ptr resulted in memleak.

src/search_engine.cpp

@@ -589,6 +589,7 @@ void SearchEngine::Search()
 				found = copy.GetComment() == s.GetComment();
 		}
 		
+		if (CaseSensitive || MatchToPattern)
 			copy.NullMe();
 		(*it)->CopyPtr(0);