XrdOucUtils::GroupName not properly checking return value of getgrgid_r

[mgmarino]

There are 2 issues:

1. The code is currently checking if "errno" is == ERANGE, but getgrgid_r is thread safe and returns the error code.
2. Even if getgrgid_r succeeds, the code needs to check the value of gEnt since it may be NULL.

From http://pubs.opengroup.org/onlinepubs/009695299/functions/getgrgid.html

int getgrgid_r(gid_t gid, struct group _grp, char *buffer,
size_t bufsize, struct group *_result);
...
The getgrgid_r() function shall update the group structure pointed to by grp and store a pointer to that structure at the location pointed to by result.... A NULL pointer shall be returned at the location pointed to by result on error or if the requested entry is not found.