tools: Fix comparison in virLoginShellGetShellArgv
tools: Fix comparison in virLoginShellGetShellArgv

Commit id '740e4d70' altered the logic to fetch the sysconf values and
added a new virConfGetValueStringList which returns -1 on failure, 0 if
missing, and 1 if the value was present.

However, the caller only checked !shargv which caught Coverity's attention
since the following VIR_ALLOC_N(*shargv, 2) would be a NULL ptr deref

Signed-off-by: John Ferlan <87558058f6f829e5ec976c8ef960720af4ff9c7d@redhat.com>

nertpinx/libvirt,VenkatDatta/libvirt,VenkatDatta/libvirt,jardasgit/libvirt,olafhering/libvirt,andreabolognani/libvirt,olafhering/libvirt,andreabolognani/libvirt,jfehlig/libvirt,libvirt/libvirt,taget/libvirt,zippy2/libvirt,zippy2/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,olafhering/libvirt,crobinso/libvirt,VenkatDatta/libvirt,crobinso/libvirt,datto/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,olafhering/libvirt,nertpinx/libvirt,jfehlig/libvirt,datto/libvirt,jfehlig/libvirt,libvirt/libvirt,eskultety/libvirt,zippy2/libvirt,crobinso/libvirt,jfehlig/libvirt,datto/libvirt,andreabolognani/libvirt,eskultety/libvirt,nertpinx/libvirt,eskultety/libvirt,VenkatDatta/libvirt,datto/libvirt,nertpinx/libvirt,taget/libvirt,fabianfreyer/libvirt,libvirt/libvirt,taget/libvirt,nertpinx/libvirt,andreabolognani/libvirt,libvirt/libvirt,zippy2/libvirt,eskultety/libvirt,jardasgit/libvirt,datto/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,taget/libvirt,fabianfreyer/libvirt,taget/libvirt,crobinso/libvirt,jardasgit/libvirt,eskultety/libvirt