Zero resources might crash the raylet

[pcmoritz]

The crash can be reproduced with the following script (this is on 0.7.0 from pip):

import ray

ray.init(num_cpus=0)

@ray.remote(num_cpus=0) 
class A(object): 
    def __init__(self): 
        pass 
    def f(self): 
        pass

a = A.remote()
x = a.f.remote()
ray.get(x)

The last line crashes the raylet with

WARNING: Logging before InitGoogleLogging() is written to STDERR
I0529 21:13:02.770886 18815 stats.h:48] Succeeded to initialize stats: exporter address is 127.0.0.1:8888
terminate called after throwing an instance of 'std::out_of_range'
  what():  _Map_base::at
*** Aborted at 1559164424 (unix time) try "date -d @1559164424" if you are using GNU date ***
PC: @                0x0 (unknown)
*** SIGABRT (@0x3e80000497f) received by PID 18815 (TID 0x7f9b6a960740) from PID 18815; stack trace: ***
    @     0x7f9b6a540890 (unknown)
    @     0x7f9b69634e97 gsignal
    @     0x7f9b69636801 abort
    @     0x7f9b6a0298b7 (unknown)
    @     0x7f9b6a02fa06 (unknown)
    @     0x7f9b6a02fa41 std::terminate()
    @     0x7f9b6a02fc74 __cxa_throw
    @     0x7f9b6a02b78f (unknown)
    @           0x45800a std::__detail::_Map_base<>::at()
    @           0x465ae2 ray::raylet::NodeManager::AssignTask()
    @           0x466407 ray::raylet::NodeManager::DispatchTasks()
    @           0x46ef6a ray::raylet::NodeManager::ProcessGetTaskMessage()
    @           0x471b6b ray::raylet::NodeManager::ProcessClientMessage()
    @           0x43011d _ZNSt17_Function_handlerIFvSt10shared_ptrIN3ray16ClientConnectionIN5boost4asio5local15stream_protocolEEEElPKhEZNS1_6raylet6Raylet12HandleAcceptERKNS3_6system10error_codeEEUlS8_lSA_E0_E9_M_invokeERKSt9_Any_dataS8_lSA_
    @           0x4e21a0 ray::ClientConnection<>::ProcessMessage()
    @           0x4de5aa boost::asio::detail::read_op<>::operator()()
    @           0x4de6f3 boost::asio::detail::reactive_socket_recv_op<>::do_complete()
    @           0x42497d boost::asio::detail::scheduler::run()
    @           0x40e041 main
    @     0x7f9b69617b97 __libc_start_main
    @           0x41f571 (unknown)