Fix zero-size malloc and memcpy

[rdeits-bd]

When running some our internal code under valgrind, I ran into warnings inside Mujoco's set0 function:

==720211== Thread 2:
==720211== Unsafe allocation with size of zero is implementation-defined
==720211==    at 0x4901C8B: memalign (vg_replace_malloc.c:2026)
==720211==    by 0x4A42AE7: mju_malloc (in /home/BOSDYN/rdeits/Projects/mujoco/build/lib/libmujoco.so.3.4.1)
==720211==    by 0x4A30CDC: set0 (in /home/BOSDYN/rdeits/Projects/mujoco/build/lib/libmujoco.so.3.4.1)
==720211==    by 0x4A336B2: mj_setConst (in /home/BOSDYN/rdeits/Projects/mujoco/build/lib/libmujoco.so.3.4.1)
==720211==    by 0x4AD6AB6: mjCModel::TryCompile(mjModel_*&, mjData_*&, mjVFS_ const*) (in /home/BOSDYN/rdeits/Projects/mujoco/build/lib/libmujoco.so.3.4.1)
==720211==    by 0x4AD2FA2: mjCModel::Compile(mjVFS_ const*, mjModel_**) (in /home/BOSDYN/rdeits/Projects/mujoco/build/lib/libmujoco.so.3.4.1)
==720211==    by 0x4B2CAEB: mj_loadXML (in /home/BOSDYN/rdeits/Projects/mujoco/build/lib/libmujoco.so.3.4.1)
==720211==    by 0x40123D8: (anonymous namespace)::LoadModel(char const*, mujoco::Simulate&) (in /home/BOSDYN/rdeits/Projects/mujoco/build/bin/simulate)
==720211==    by 0x40172EF: PhysicsThread(mujoco::Simulate*, char const*) (in /home/BOSDYN/rdeits/Projects/mujoco/build/bin/simulate)
==720211==    by 0x4DF2252: ??? (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.30)
==720211==    by 0x50DFAC2: start_thread (pthread_create.c:442)
==720211==    by 0x5170A83: clone (clone.S:100)

I can reproduce this by running the simulate binary after building from source:

~/apps/valgrind-3.26.0/build/bin/valgrind ./bin/simulate <my robot model.xml>

The issue is that m->nflex can be zero, in which case we end up calling aligned_alloc with a size of zero. I can't find clear information on whether that's allowed or not -- some sources say yes, others say no. The memcpy on the subsequent line is also suspect: If malloc returns null (which I think it's allowed to for zero size), then even a zero-size memcpy is undefined behavior with a null destination.

This change resolves the valgrind warning in set0. Note that we do still see some warnings inside libnvidia-glcore both before and after this change.

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[quagla]

Thanks for the change! A small remark, can you please move the comments from inside the if to the start of the blocks that you changed?

[rdeits-bd]

Happy to make that change. I'm currently blocked waiting for our lawyers to approve your CLA. If you'd rather just make the relevant commit yourself that's of course fine by me.