[tools] Fix grokdump.py D/CHECK error message extraction

Change-Id: I5ee7de89a262fce7db25a970a3c56c25eb6f6803
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/958924
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51879}

tools/grokdump.py

@@ -2163,6 +2163,7 @@ class InspectionPadawan(object):
     return None
 
   def TryExtractErrorMessage(self, slot, start, end, print_message):
+    ptr_size = self.reader.PointerSize()
     end_marker = ERROR_MESSAGE_MARKER + 1;
     header_size = 1
     end_search = start + 1024 + (header_size * ptr_size);