[tools] Make grokdump robust against non-readable fields

Change-Id: Idb96a923e5cdb250c0b0e3240316cbcfab963108
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/6586929
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#100497}

tools/grokdump.py

@@ -1225,16 +1225,22 @@ class HeapObject(object):
                            instance_type)
 
   def ObjectField(self, offset):
+    if not self.heap.reader.IsValidAddress(self.address + offset):
+      return None
     field_value = self.heap.reader.ReadTagged(self.address + offset)
     return self.heap.FindObjectOrSmi(field_value)
 
   def SmiField(self, offset):
+    if not self.heap.reader.IsValidAddress(self.address + offset):
+      return None
     field_value = self.heap.reader.ReadTagged(self.address + offset)
     if self.heap.IsSmi(field_value):
       return self.heap.SmiUntag(field_value)
     return None
 
   def Uint32Field(self, offset):
+    if not self.heap.reader.IsValidAddress(self.address + offset):
+      return None
     return self.heap.reader.ReadU32(self.address + offset)
 
 
@@ -1428,7 +1434,9 @@ class ConsString(String):
 
   def GetChars(self):
     try:
-      return self.left.GetChars() + self.right.GetChars()
+      left = self.left.GetChars() if self.left is not None else "<lhs>"
+      right = self.right.GetChars() if self.right is not None else "<rhs>"
+      return left + right
     except Exception as e:
       return "***CAUGHT EXCEPTION IN GROKDUMP***: " + str(e)