stupidgcm: NewChacha20poly1305: avoid slice append

I noticed that growslice() shows up in the cpuprofile.
Avoiding slice append for the private jey copy gives a 0.6% speedup:

gocryptfs/internal/speed$ benchstat old new
name             old time/op   new time/op   delta
StupidXchacha-4   5.68µs ± 0%   5.65µs ± 0%  -0.63%  (p=0.008 n=5+5)

name             old speed     new speed     delta
StupidXchacha-4  721MB/s ± 0%  725MB/s ± 0%  +0.63%  (p=0.008 n=5+5)

0 files changed, 0 insertions, 0 deletions