mysqli field_count nullref check

ref: #1162

Author: jakubmisek

src/Peachpie.Library.MySql/MySqli/mysqli.cs

@@ -88,7 +88,7 @@ internal mysqli(Context ctx, IDbConnection dbconnection)
         /// <summary>
         /// Returns the number of columns for the most recent query.
         /// </summary>
-        public int field_count => Connection.LastResult.FieldCount;
+        public int field_count => Connection.LastResult?.FieldCount ?? 0;
 
         /// <summary>
         /// Get MySQL client info.