[CDC] SIGSEGV due to null consensus pointer in GetChanges

[ajcaldera1]

Found during CDC testing on producer node:

#0 0x00007fa7a002f9f3 in yb::cdc::CDCServiceImpl::GetChanges (this=this@entry=0x281a2c0, req=req@entry=0x48db910, resp=resp@entry=0xa5c2dd60, context=...) at ../../ent/src/yb/cdc/cdc_service.cc:367
#1 0x00007fa79fdc1b9b in yb::cdc::CDCServiceIf::Handle (this=0x281a2c0, call=...) at src/yb/cdc/cdc_service.service.cc:142
#2 0x00007fa79a17dcd9 in yb::rpc::ServicePoolImpl::Handle (this=0x28fdb00, incoming=...) at ../../src/yb/rpc/service_pool.cc:262
#3 0x00007fa79a121f44 in yb::rpc::InboundCall::InboundCallTask::Run (this=) at ../../src/yb/rpc/inbound_call.cc:212
#4 0x00007fa79a189928 in yb::rpc::(anonymous namespace)::Worker::Execute (this=) at ../../src/yb/rpc/thread_pool.cc:99
#5 0x00007fa79873857f in operator() (this=0xb3465f8) at /home/yugabyte/yb-software/yugabyte-2.0.10.0-217ffbcec74db/Cellar/gcc/5.5.0_4/include/c++/5.5.0/functional:2267
#6 yb::Thread::SuperviseThread (arg=0xb3465a0) at ../../src/yb/util/thread.cc:739
#7 0x00007fa79317e694 in start_thread (arg=0x7fa6eb5f2700) at pthread_create.c:333
#8 0x00007fa7928bb41d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:109

[hectorgcr]

This is related to the line

tablet_peer->consensus()->UpdateCDCConsumerOpId(GetMinSentCheckpointForTablet(req->tablet_id()));

TabletPeer::consensus returns a raw pointer, and this pointer was null:

has_consensus_ = {_M_base = {static _S_alignment = 1, _M_i = false}}
consensus_ = {<std::__shared_ptr<yb::consensus::RaftConsensus, (__gnu_cxx::_Lock_policy)2>> = {_M_ptr = 0x0, _M_refcount = {_M_pi = 0x0}}

We need to change consensus() to shared_consensus()