g10: fix uninit use in aead filter.

* g10/decrypt-data.c (aead_underflow): Initialize 'err'
  to 0.

Just initialize to 0 and then there's no risk of using an uninit var
if we (somehow) fall through to the end label.

Found by GCC's -fanalyzer.

• Signed-off-by: Sam James <sam@gentoo.org>