Fix buffer overrun in gettag for GCM

* cipher/cipher-gcm.c: copy a fixed length instead of the user-supplied
  number.

The outbuflen is used to check the minimum size, the real tag is always
of fixed length.

Actually this is not a buffer overrun because we copy not more than
has been allocated for OUTBUF. However a too long OUTBUFLEN accesses
data outside of the source buffer. -wk

• Signed-off-by: Peter Wu <peter@lekensteyn.nl>