hdspmixer: fix null pointer dereference in HDSPMixerWindow:handler_cb

alsa-project/alsa-tools pull request #42 was edited from cheldt:

Fixes following error: Program received signal SIGSEGV, Segmentation fault. handler_cb () at /home/snaut/dev/alsa-tools/hdspmixer/src/HDSPMixerWindow.cxx:278

Related [Issue](https://github.com/alsa-project/alsa-tools/issues/43)

Request URL : https://github.com/alsa-project/alsa-tools/pull/42 Patch URL : https://github.com/alsa-project/alsa-tools/pull/42.patch Repository URL: https://github.com/alsa-project/alsa-tools