audiopolicy: engineconfigurable: fix potential segfault if no default device

If no default device is available from configuration.xml, and if
the engine fallbacks on it anyway, it tries to get its type without checking
the pointer validity.

This CL fix the potential segfault and assert with explicit error message.

Test: build

Change-Id: Icf8d7f5ef998dad6f4033d934b48d408030c7e17
Signed-off-by: François Gaffie <francois.gaffie@renault.com>