Don't return stack memory in fr_getgrnam
    
    This fixes the following Coverity issue:
    
        Error: RETURN_LOCAL (CWE-562):
        freeradius-server-3.0.4/src/modules/rlm_unix/rlm_unix.c:87: local_ptr_identity_local: "getgrnam_r(name, &my_group, group_buffer, group_size, &grp)" stores "&my_group" (address of local variable "my_group") into "grp".
        freeradius-server-3.0.4/src/modules/rlm_unix/rlm_unix.c:99: return_local_addr_alias: Returning pointer "grp" which points to local variable "my_group".
    
    Resolves: Bug#1120234