Fix in SoftAVCEncoder plugin

size of buffer_handle_t, a pointer type, was assumed to be
4 bytes. This patch uses sizeof(buffer_handle_t) instead.

In addition, replaced magic number 4 with sizeof(OMX_U32)

Change-Id: Id5a7ed805a7b8e9867d8e51655cd334a66ac2c2b
Signed-off-by: Ashok Bhat <ashok.bhat@arm.com>