Check that 'M/Counter' exists before trying to read it.

Some old zim files may not have a 'Counter` metadata article.
We have to handle this correctly.

src/reader.cpp

@@ -103,6 +103,7 @@ namespace kiwix {
 
     zim::Article article = this->zimFileHandler->getArticle('M',"Counter");
 
+    if ( article.good() ) {
       stringstream ssContent(article.getData());
 
       while(getline(ssContent, item,  ';')) {
@@ -114,6 +115,7 @@ namespace kiwix {
 	  counters.insert(pair<string, int>(mimeType, counter));
         }
       }
+    }
 
     return counters;
   }