ALSA: usb-audio: Check for possible chip NULL pointer before clearing probing flag

Before clearing the probing flag in the error exit path, check that the
chip pointer is not NULL.

Signed-off-by: Thomas Pfaff <tpfaff@gmx.net>
Cc: <stable@kernel.org> [2.6.39+]
Signed-off-by: Takashi Iwai <tiwai@suse.de>