softmixer/bluetooth: fails to open softvol control at startup

From time to time,when a phone call is done, the softmixer fails to create the SCO listening or SCO talk streams, complaining about a softvol mismatch.

By reading the code of libasound, the only possible reason is that a character string has been crashed by a memory overlap.

This is under investigation by running it with valgrind, and seems hard to reproduce.