libmedia: add NULL check before accessing source and destination buffers.

IMemory can return a NULL pointer or the destination buffer for readAt can
be NULL. Check for NULL and report error.

Author:    Shivaprasad Hongal <shongal@codeaurora.org>

Change-Id: I6df9275f2eb9a6e1c2641d084d713b73737d68d7