Consistent error checking for sp<IMemory> and pointer()

There have been concerns that an sp<IMemory> could be non-0, but the
associated pointer() still be NULL.  There are rumors this may happen
when a non-0 sp<IMemory> is passed in by client but the shared memory cannot
be re-mapped into mediaserver.

There's also evidence in the early (2009/03/03) pre-git code of checking
pointer() for NULL, after a local allocate() returned a non-0 sp<IMemory>.
It's not clear if this is "cargo cult" paranoia, or if there was a
genuine reason for the check.

In any case, we now consistently check pointer() for sp<IMemory>
input parameters in createTrack() and queueTimedBuffer().

We also check after successful allocate().  If allocate() returns a
non-0 sp<> but NULL pointer(), then treat it as if the allocate() had
returned 0.

Change-Id: I3013ac5766b493d443ecef71711ec861076a623e