Null pointer dereference in eigerDetector::writeOctet if name for user's GID is missing

[nmarks99]

In the presumably rare situation where a user's owning group is not found, e.g.:

$ id -gn
id: cannot find name for group ID 3468
3468

eigerDetector::writeOctet causes a segmentation fault due to dereferencing a null pointer which is returned from getgrgid() if the group isn't found. The segfault occurs shortly after IOC init and there is no helpful error message to diagnose the problem.

ADEiger/eigerApp/src/eigerDetector.cpp

Line 728 in 7f52423

mFileOwnerGroup->put(getgrgid(mFsGid)->gr_name);

I tested this situation with a minimal example program and compared the results for an account with a valid group and another with an invalid group ID:

// file: check_gid.cpp
#include <iostream>
#include <unistd.h>
#include <grp.h>
int main() {
    struct group* p_group = getgrgid(getgid());
    std::cout << p_group->gr_name << std::endl;

# Account with valid gid
$ id -gn
aesbc
$ ./check_gid
aesbc

# Account with invalid gid
$ id -gn
id: cannot find name for group ID 3468
3468
$ ./check_gid
Segmentation fault (core dumped)