audiopolicy: add null pointer check for primary hw

When sound card detection fails, primary HW cannot be opened.
add null pointer check to avoid further crashes in audioserver.

Bug: 136060200
Test: make
authored-by: Weiyin Jiang <wjiang@codeaurora.org>
Change-Id: Ief07a36ca8aed0e6013d533bd0aa66e122e3070c