Initialize bwr.read_buffer in IPCThreadState::talkWithDriver.

I/valgrind( 1309): ==1310== Syscall param
ioctl(BINDER_WRITE_READ).read_buffer points to uninitialised byte(s)
I/valgrind( 1309): ==1310==    at 0x480E670: __ioctl (__ioctl.S:10)
I/valgrind( 1309): ==1310==  Address 0xbda651b4 is on thread 1's stack

Change-Id: I02893df7b5786b6b2dbd9659f5706d7171295ab2