Check that the stream is valid, if specified at all, and return false if
it isn't -- or if no size was specified and determining stream size
failed.
Check for SetData() success in the test to provide better diagnostics in
case the file it uses is not found (as is the case when running the test
from another directory, for example).
Also pass wxSharedPtr<> by const reference instead of by value to avoid
unnecessary copies.
