used buffer instead of string concatenation

with buffer :
time (for i in {0..10000}; do ./pastek_core.native < tmp > /dev/null;done)
real	0m45.771s
user	0m7.144s
sys	0m5.296s

with string concat:
time (for i in {0..10000}; do pastek_core < tmp > /dev/null;done)
real	0m52.498s
user	0m26.540s
sys	0m23.444s