[OpenMP][FIX] Do not dereference a potential nullptr

[jdoerfert]

Summary

The first thread state in the new GPU runtime doesn't have a previous
one and we should not dereference the nullptr placeholder.

openmp/libomptarget/DeviceRTL/src/State.cpp

struct ThreadStateTy {
 
   ThreadStateTy *PreviousThreadState;
 
   void init() {
     ICVState = TeamState.ICVState;
     PreviousThreadState = nullptr;
   }
 
-  void init(ThreadStateTy &PreviousTS) {
-    ICVState = PreviousTS.ICVState;
-    PreviousThreadState = &PreviousTS;
+  void init(ThreadStateTy *PreviousTS) {
+    ICVState = PreviousTS ? PreviousTS->ICVState : TeamState.ICVState;
+    PreviousThreadState = PreviousTS;
   }
 };
 
 __attribute__((loader_uninitialized))
 ThreadStateTy *ThreadStates[mapping::MaxThreadsPerTeam];
 #pragma omp allocate(ThreadStates) allocator(omp_pteam_mem_alloc)
 
 uint32_t &lookupForModify32Impl(uint32_t ICVStateTy::*Var) {
void state::init(bool IsSPMD) {
 
   ThreadStates[mapping::getThreadIdInBlock()] = nullptr;
 }
 
 void state::enterDataEnvironment() {
   unsigned TId = mapping::getThreadIdInBlock();
   ThreadStateTy *NewThreadState =
       static_cast<ThreadStateTy *>(__kmpc_alloc_shared(sizeof(ThreadStateTy)));
-  NewThreadState->init(*ThreadStates[TId]);
+  NewThreadState->init(ThreadStates[TId]);
   ThreadStates[TId] = NewThreadState;
 }
 
 void state::exitDataEnvironment() {
   unsigned TId = mapping::getThreadIdInBlock();
   resetStateForThread(TId);
 }