ALSA: usb: scarlett2: Fix missing NULL check

scarlett2_input_select_ctl_info() sets up the string arrays allocated
via kasprintf(), but it misses NULL checks, which may lead to NULL
dereference Oops.  Let's add the proper NULL check.

Fixes: 8eba063b ("ALSA: scarlett2: Simplify linked channel handling")
Link: https://patch.msgid.link/20250731053714.29414-1-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>