ALSA: aloop: Avoid pointer dereference before null-check

ALSA: aloop: Avoid pointer dereference before null-check

Static analysis tools (cppcheck and PVS Studio) report an error
in loopback_snd_timer_period_elapsed() regarding dpcm_play pointer
dereference earlier than its null-check. And although this is a result
of a formal check, and the pointer correctness is also protected
by having a corresponding bit set in the "running" mask, re-ordering
of the lines can imake the code even formally correct and eliminate
those static analysis error reports.

Fixes: 26c53379f98d ("ALSA: aloop: Support selection of snd_timer instead of jiffies")
Reported-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Signed-off-by: Andrew Gabbasov <andrew_gabbasov@mentor.com>
Link: https://lore.kernel.org/r/20191127110622.26105-1-andrew_gabbasov@mentor.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>