Add support for reading nop operations

Previously this fell through to a naked codet("nop"). This would crash
in the symex phase. This correctly converts it into an appropriate skip

Author: thk123

src/java_bytecode/java_bytecode_convert_method.cpp

@@ -2324,6 +2324,10 @@ codet java_bytecode_convert_methodt::convert_instructions(
       results[1]=op[0];
       results[0]=op[1];
     }
+    else if(statement=="nop")
+    {
+      c=code_skipt();
+    }
     else
     {
       c=codet(statement);