Fix a set of minor DAC bugs I encountered recently (dotnet#100031)

In src/coreclr/debug/daccess/dacdbiimpl.cpp
- If g_pDebugger is NULL, produce an error, or NULL result instead of an invalid memory access.
- This caused problems when debugging with a debug version of the DAC

In src/coreclr/debug/di/module.cpp
- Avoid attempting to get the MapAddr if the number of entries in the map is 0, as it may not yet be fully initialized and attempting to call the GetMapAddr function may cause an ASSERT in debug/checked versions of the DAC

In src/coreclr/utilcode/collections.cpp
- If a CHashTable is not fully initialized, or is in an intermediate state, its possible for the linked list to have an infinite cycle. Tweak the iteration here under the DAC to stop
  - This is  known to cause stack walking to loop infinitely in the debugger when debugging a runtime which has suspended at a non-safe point

src/coreclr/debug/daccess/dacdbiimpl.cpp

@@ -5627,10 +5627,13 @@ void DacDbiInterfaceImpl::LookupEnCVersions(Module*          pModule,
     DebuggerJitInfo * pDJI = NULL;
     EX_TRY_ALLOW_DATATARGET_MISSING_MEMORY
     {
-        pDMI = g_pDebugger->GetOrCreateMethodInfo(pModule, mdMethod);
-        if (pDMI != NULL)
+        if (g_pDebugger != NULL)
         {
-            pDJI = pDMI->FindJitInfo(pMD, CORDB_ADDRESS_TO_TADDR(pNativeStartAddress));
+            pDMI = g_pDebugger->GetOrCreateMethodInfo(pModule, mdMethod);
+            if (pDMI != NULL)
+            {
+                pDJI = pDMI->FindJitInfo(pMD, CORDB_ADDRESS_TO_TADDR(pNativeStartAddress));
+            }
         }
     }
     EX_END_CATCH_ALLOW_DATATARGET_MISSING_MEMORY;
@@ -7479,6 +7482,10 @@ HRESULT DacDbiInterfaceImpl::GetDefinesBitField(ULONG32 *pDefines)
     DD_ENTER_MAY_THROW;
     if (pDefines == NULL)
         return E_INVALIDARG;
+
+    if (g_pDebugger == NULL)
+        return CORDBG_E_NOTREADY;
+
     *pDefines = g_pDebugger->m_defines;
     return S_OK;
 }
@@ -7488,6 +7495,10 @@ HRESULT DacDbiInterfaceImpl::GetMDStructuresVersion(ULONG32* pMDStructuresVersio
     DD_ENTER_MAY_THROW;
     if (pMDStructuresVersion == NULL)
         return E_INVALIDARG;
+
+    if (g_pDebugger == NULL)
+        return CORDBG_E_NOTREADY;
+
     *pMDStructuresVersion = g_pDebugger->m_mdDataStructureVersion;
     return S_OK;
 }

src/coreclr/debug/di/module.cpp

@@ -4250,12 +4250,12 @@ HRESULT CordbNativeCode::GetILToNativeMapping(ULONG32                    cMap,
         LoadNativeInfo();
 
         SequencePoints * pSeqPts = GetSequencePoints();
-        DebuggerILToNativeMap * rgMapInt = pSeqPts->GetMapAddr();
         ULONG32 cMapIntCount = pSeqPts->GetEntryCount();
 
         // If they gave us space to copy into...
-        if (map != NULL)
+        if (map != NULL && cMapIntCount != 0)
         {
+            DebuggerILToNativeMap * rgMapInt = pSeqPts->GetMapAddr();
             // Only copy as much as either they gave us or we have to copy.
             ULONG32 cMapToCopy = min(cMap, cMapIntCount);
 

src/coreclr/utilcode/collections.cpp

@@ -268,6 +268,12 @@ BYTE *CHashTable::FindNextEntry(        // The next entry, or0 for end of list.
         if (psSrch->iNext != UINT32_MAX)
         {
             psEntry = EntryPtr(psSrch->iNext);
+#if DACCESS_COMPILE
+            // If there is a simple infinite loop in the linked list
+            // If more complex forms of infinite loops are present, this code may need to be adjusted to handle an arbitrary cycle.
+            if (psEntry->iNext == psSrch->iNext)
+                return NULL;
+#endif
             psSrch->iNext = psEntry->iNext;
             return ((BYTE *) psEntry);
         }