Fix memory leakage due to unreleased MuxOMX objects after disconnection

o related-to-bug: 5984232

Change-Id: I20cbeff6ae865e995d02342549f1e09320543740
diff --git a/media/libstagefright/OMXClient.cpp b/media/libstagefright/OMXClient.cpp
index 7a805aa..7cdb793 100644
--- a/media/libstagefright/OMXClient.cpp
+++ b/media/libstagefright/OMXClient.cpp
@@ -335,6 +335,10 @@
 }
 
 void OMXClient::disconnect() {
+    if (mOMX.get() != NULL) {
+        mOMX.clear();
+        mOMX = NULL;
+    }
 }
 
 }  // namespace android