#2450 Segfault on s390x with 5.4.1 during docs build

[Scott Talbert]

Observing a segmentation fault when trying to build the documentation on s390x. Segfault occurs when running:

GNUPLOT_LIB=../demo ../src/gnuplot -e "winhelp=1;" ./plotstyles.gnu

Backtrace:

Core was generated by `../src/gnuplot -e winhelp=1; ./plotstyles.gnu'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x000002aa1ea9aeaa in tessellate_one_cube (iz=5, iy=<optimized out>, 
    ix=<optimized out>, plot=<optimized out>) at ../../src/vplot.c:320
320             quad[3].x = intersection[ivertex][0];
(gdb) bt
#0  0x000002aa1ea9aeaa in tessellate_one_cube (iz=5, iy=<optimized out>, 
    ix=<optimized out>, plot=<optimized out>) at ../../src/vplot.c:320
#1  vplot_isosurface (plot=<optimized out>, downsample=<optimized out>)
    at ../../src/vplot.c:213
#2  0x000002aa1e9d3310 in do_3dplot (plots=0x2aa1f00bfb0, 
    pcount=<optimized out>, replot_mode=<optimized out>)
    at ../../src/graph3d.c:1105
#3  0x000002aa1ea0cdac in eval_3dplots () at ../../src/plot3d.c:2755
#4  0x000002aa1e9abb0c in splot_command () at ../../src/command.c:2308
#5  0x000002aa1e9a8a0c in command () at ../../src/command.c:659
#6  do_line () at ../../src/command.c:429
#7  0x000002aa1e9f07c8 in load_file (fp=0x2aa1ed38e20, name=<optimized out>, 
    calltype=<optimized out>) at ../../src/misc.c:335
#8  0x000002aa1e99d0aa in main (argc_orig=<optimized out>, argv=0x3ffe5f7e4d0)
    at ../../src/plot.c:688
(gdb) 

Note that s390x is big endian.

[Ethan Merritt]

Same as #2467