IOMX.cpp uninitialized pointer in BnOMX::onTransact

This can lead to local code execution in media server.
Fix initializes the pointer and checks the error conditions before
returning

Bug: 26403627
Ticket: CYNGNOS-2213
Change-Id: I7fa90682060148448dba01d6acbe3471d1ddb500
(cherry picked from commit 9d72eb06)