Optimised map

[Peter Bortas @ Pike developers forum]

I threw your branch on the Pike-experimental Coverity stream. You
might want to have a look at CID 1358354:


*** CID 1358354:  Null pointer dereferences  (FORWARD_NULL)
/home/covscan/pike/pike-git/src/builtin.cmod: 4280 in low_automap()
4274            push_svalue(ITEM(tmpargs[e].u.array)+x);
4275           }else{
4276            push_svalue(tmpargs+e);
4277           }
4278         }
4279     
>>>     CID 1358354:  Null pointer dereferences  (FORWARD_NULL)
>>>     Comparing "reuse_ctx" to null implies that "reuse_ctx" might be null.
4280         if(reuse_ctx != NULL)
4281           apply_svalue_reuse_context (reuse_ctx);
4282         else
4283           low_automap(d+1,depth,fun,real_args,args);
4284         stack_pop_to_no_free (ITEM(ret) + x);
4285         types |= 1 << TYPEOF(ITEM(ret)[x]);