tools/libs/light: don't touch nr_vcpus_out if listing vcpus and returning NULL

tools/libs/light: don't touch nr_vcpus_out if listing vcpus and returning NULL

If we are in libxl_list_vcpu() and we are returning NULL, let's avoid
touching the output parameter *nr_vcpus_out, which the caller should
have initialized to 0.

The current behavior could be problematic if are creating a domain and,
in the meantime, an existing one is destroyed when we have already done
some steps of the loop. At which point, we'd return a NULL list of vcpus
but with something different than 0 as the number of vcpus in that list.
And this can cause troubles in the callers (e.g., nr_vcpus_on_nodes()),
when they do a libxl_vcpuinfo_list_free().

Crashes due to this are rare and difficult to reproduce, but have been
observed, with stack traces looking like this one:

#0  libxl_bitmap_dispose (map=map@entry=0x50) at libxl_utils.c:626
#1  0x00007fe72c993a32 in libxl_vcpuinfo_dispose (p=p@entry=0x38) at _libxl_types.c:692
#2  0x00007fe72c94e3c4 in libxl_vcpuinfo_list_free (list=0x0, nr=<optimized out>) at libxl_utils.c:1059
#3  0x00007fe72c9528bf in nr_vcpus_on_nodes (vcpus_on_node=0x7fe71000eb60, suitable_cpumap=0x7fe721df0d38, tinfo_elements=48, tinfo=0x7fe7101b3900, gc=0x7fe7101bbfa0) at libxl_numa.c:258
#4  libxl__get_numa_candidate (gc=gc@entry=0x7fe7100033a0, min_free_memkb=4233216, min_cpus=4, min_nodes=min_nodes@entry=0, max_nodes=max_nodes@entry=0, suitable_cpumap=suitable_cpumap@entry=0x7fe721df0d38, numa_cmpf=0x7fe72c940110 <numa_cmpf>, cndt_out=0x7fe721df0cf0, cndt_found=0x7fe721df0cb4) at libxl_numa.c:394
#5  0x00007fe72c94152b in numa_place_domain (d_config=0x7fe721df11b0, domid=975, gc=0x7fe7100033a0) at libxl_dom.c:209
#6  libxl__build_pre (gc=gc@entry=0x7fe7100033a0, domid=domid@entry=975, d_config=d_config@entry=0x7fe721df11b0, state=state@entry=0x7fe710077700) at libxl_dom.c:436
#7  0x00007fe72c92c4a5 in libxl__domain_build (gc=0x7fe7100033a0, d_config=d_config@entry=0x7fe721df11b0, domid=975, state=0x7fe710077700) at libxl_create.c:444
#8  0x00007fe72c92de8b in domcreate_bootloader_done (egc=0x7fe721df0f60, bl=0x7fe7100778c0, rc=<optimized out>) at libxl_create.c:1222
#9  0x00007fe72c980425 in libxl__bootloader_run (egc=egc@entry=0x7fe721df0f60, bl=bl@entry=0x7fe7100778c0) at libxl_bootloader.c:403
#10 0x00007fe72c92f281 in initiate_domain_create (egc=egc@entry=0x7fe721df0f60, dcs=dcs@entry=0x7fe7100771b0) at libxl_create.c:1159
#11 0x00007fe72c92f456 in do_domain_create (ctx=ctx@entry=0x7fe71001c840, d_config=d_config@entry=0x7fe721df11b0, domid=domid@entry=0x7fe721df10a8, restore_fd=restore_fd@entry=-1, send_back_fd=send_back_fd@entry=-1, params=params@entry=0x0, ao_how=0x0, aop_console_how=0x7fe721df10f0) at libxl_create.c:1856
#12 0x00007fe72c92f776 in libxl_domain_create_new (ctx=0x7fe71001c840, d_config=d_config@entry=0x7fe721df11b0, domid=domid@entry=0x7fe721df10a8, ao_how=ao_how@entry=0x0, aop_console_how=aop_console_how@entry=0x7fe721df10f0) at libxl_create.c:2075

Signed-off-by: Dario Faggioli <dfaggioli@suse.com>
Tested-by: James Fehlig <jfehlig@suse.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>