Fix a segfault in AudioFlinger.

Check the string returned by a HAL's implementation of get_parameters
for NULL before attempting to make use of it.  That way, we won't
bring down the mediaserver because of a poorly written HAL.

Change-Id: Ic99d7b004520d7d6347842a681c0595e889b68ea
Signed-off-by: John Grossman <johngro@google.com>