AdvLoggerPkg: Minor fix in DecodeUefiLog.py _GetNextMessageBlock()

The function calls `self._ReadMessageEntry()` to read
the next message entry from the log file. That function can return
`None` for the `MessageEntry` if it encounters an invalid signature.

This function needs to handle that case properly to avoid trying to
access fields of a `None` object, which would lead to an
`AttributeError`.

Signed-off-by: Michael Kubacki <[email protected]>

Author: makubacki

AdvLoggerPkg/Application/DecodeUefiLog/DecodeUefiLog.py

@@ -588,6 +588,10 @@ def _GetNextMessageBlock(self, LoggerInfo):
 
         (MessageEntry, NextMessage) = self._ReadMessageEntry(LoggerInfo)
 
+        if MessageEntry is None:
+            print("Invalid message signature at offset 0x%x" % InFile.tell())
+            raise Exception("Message block has the wrong signature at offset 0x%x" % InFile.tell())
+
         if MessageEntry["Signature"] != 'ALMS' and MessageEntry["Signature"] != 'ALM2':
             print("Log signature was incorrect.  Should be either 'ALMS' or 'ALM2', was '%s'" % MessageEntry["Signature"])
             raise Exception("Message Block has wrong signature at offset 0x%X" % InFile.tell())