Bug 1272371 - Update multiplexed input stream seek behaviour. r=froydnj

This patch is originally from :baku and updates the seek behaviour of
nsMulitplexInputStream. This fixes an issue with absolute seeking where if a
seek is performed and the correct offset reached, the members of the multiplex
stream were not updated to reflect this.

MozReview-Commit-ID: 7lEvuXxzYkI

--HG--
extra : rebase_source : d86a7ce0fc26abd3c9a60810bd7f7f6346e7a5b0

xpcom/io/nsMultiplexInputStream.cpp

@@ -501,7 +501,10 @@ nsMultiplexInputStream::Seek(int32_t aWhence, int64_t aOffset)
         }
       } else {
         NS_ASSERTION(remaining == streamPos, "Huh?");
+        MOZ_ASSERT(remaining != 0, "Zero remaining should be handled earlier");
         remaining = 0;
+        mCurrentStream = i;
+        mStartedReadingCurrent = true;
       }
     }