Single thread crash with PMillenium-1536

On COSMA8 running with a single thread we crash early, shortly after reading the data or during space_rebuild, depending on which checks are enabled.

With the sanitizer we fail with a segfault:

ASAN:DEADLYSIGNAL
=================================================================
==49435==ERROR: AddressSanitizer: SEGV on unknown address 0x2a73b95d115c (pc 0x0000006bdb56 bp 0x7ffc66dcc670 sp 0x7ffc66dcc180 T0)
==49435==The signal is caused by a WRITE memory access.
    #0 0x6bdb55 in space_rebuild /cosma7/data/dp004/pdraper/swiftsim-master-clean/src/space_rebuild.c:874
    #1 0x4af922 in engine_rebuild /cosma7/data/dp004/pdraper/swiftsim-master-clean/src/engine.c:1170
    #2 0x4bf8a7 in engine_init_particles /cosma7/data/dp004/pdraper/swiftsim-master-clean/src/engine.c:1710
    #3 0x40fca8 in main /cosma7/data/dp004/pdraper/swiftsim-master-clean/examples/main.c:1412
    #4 0x2b485b7a2554 in __libc_start_main (/lib64/libc.so.6+0x22554)
    #5 0x4130fb  (/cosma7/data/dp004/pdraper/swiftsim-master-clean/examples/swift+0x4130fb)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /cosma7/data/dp004/pdraper/swiftsim-master-clean/src/space_rebuild.c:874 in space_rebuild

which is the line:

 cells_top[g_index[k]].grav.count =
          k - last_gindex + 1 - space_extra_gparts;

so looks like an issue with indexing.

Running with debugging checks on we see:

[00951.6] main.c:main():1131: Linking problem

which is a check that the type of gparts isn't swift_type_gas when not using hydro. So looks like the data is read incorrectly.