ALSA: pcm: oss: Place the plugin buffer overflow checks correctly (for 5.7)

[ This is again a forward-port of the fix applied for 5.6-base code
  (commit 4285de07) to 5.7-base, hence neither Fixes nor
  Cc-to-stable tags are included here -- tiwai ]

The checks of the plugin buffer overflow in the previous fix by commit
  f2ecf903 ("ALSA: pcm: oss: Avoid plugin buffer overflow")
are put in the wrong places mistakenly, which leads to the expected
(repeated) sound when the rate plugin is involved.  Fix in the right
places.

Also, at those right places, the zero check is needed for the
termination node, so added there as well, and let's get it done,
finally.

Link: https://lore.kernel.org/r/20200424193843.20397-1-tiwai@suse.de


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