This one is a function of setting parisizemax - if parisize is initially set to 128MB, SEGV doesn't happen:
GP/PARI CALCULATOR Version 2.8.0 (development 16522-6865e62)
amd64 running linux (x86-64/GMP-5.0.5 kernel) 64-bit version
compiled: May 19 2014, gcc version 4.7.2 (Debian 4.7.2-5)
threading engine: pthread
(readline v6.2 disabled, extended help enabled)
Copyright (C) 2000-2014 The PARI Group
PARI/GP is free software, covered by the GNU General Public License, and comes
WITHOUT ANY WARRANTY WHATSOEVER.
Type ? for help, \q to quit.
Type ?12 for how to get moral (and possibly technical) support.
parisize = 8000000, primelimit = 500000, nbthreads = 24
*** Warning: new maximum stack size = 128000000 (122.070 Mbytes).
*** nfinit: Warning: increasing stack size to 16000000.
*** nfinit: Warning: increasing stack size to 32000000.
*** nfinit: Warning: increasing stack size to 64000000.
*** at top-level: nfinit([x^128-84*x^1
*** ^--------------------
*** nfinit: bug in PARI/GP (Segmentation Fault), please report.
Input file attached (I tried, but couldn't reproduce a problem with simpler input).