fix assertion about 0 msg size when first msg you select is fetched with mpod r=cavin, sr=mscott

mailnews/imap/src/nsImapServerResponseParser.cpp

@@ -2688,7 +2688,8 @@ void nsImapServerResponseParser::SetSyntaxError(PRBool error)
 
 nsresult nsImapServerResponseParser::BeginMessageDownload(const char *content_type)
 {
-		NS_ASSERTION(fSizeOfMostRecentMessage > 0, "most recent message has 0 or negative size");
+  // if we're downloading a message, assert that we know its size.
+  NS_ASSERTION(fDownloadingHeaders || fSizeOfMostRecentMessage > 0, "most recent message has 0 or negative size");
   nsresult rv = fServerConnection.BeginMessageDownLoad(fSizeOfMostRecentMessage, 
     content_type);
   if (NS_FAILED(rv))