Avoid crashing if getgrnam() cannont find a group by the given name

Without this, we get (on systems without a 'mail' group):

ERROR: Error executing a python function in /home/x/g/poky/meta/recipes-kernel/linux-libc-headers/linux-libc-headers_3.16.bb:

The stack trace of python calls that resulted in this exception/failure was:
File: 'fixup_perms', lineno: 231, function: <module>
0227: each_file = os.path.join(root, f)
0228: fix_perms(each_file, fs_perms_table[dir].fmode, fs_perms_table[dir].fuid, fs_perms_table[dir].fgid, dir)
0229:
0230:
*** 0231:fixup_perms(d)
0232:
File: 'fixup_perms', lineno: 173, function: fixup_perms
0169: if len(lsplit) != 8 and not (len(lsplit) == 3 and lsplit[1].lower() == "link"):
0170: msg = "Fixup perms: %s invalid line: %s" % (conf, line)
0171: package_qa_handle_error("perm-line", msg, d)
0172: continue
*** 0173: entry = fs_perms_entry(d.expand(line))
0174: if entry and entry.path:
0175: fs_perms_table[entry.path] = entry
0176: f.close()
0177:
File: 'fixup_perms', lineno: 32, function: __init__
File "fixup_perms", line 32, in __init__

File: 'fixup_perms', lineno: 44, function: _setdir
File "fixup_perms", line 44, in _setdir

File: 'fixup_perms', lineno: 75, function: _procgid
File "fixup_perms", line 75, in _procgid

Exception: KeyError: 'getgrnam(): name not found: mail'

ERROR: Function failed: fixup_perms
ERROR: Logfile of failure stored in: /home/x/obj/y/tmp/work/i586-poky-linux/linux-libc-headers/3.16-r0/temp/log.do_package.15841
ERROR: Task 400 (/home/x/g/poky/meta/recipes-kernel/linux-libc-headers/linux-libc-headers_3.16.bb, do_package) failed with exit code '1'

---
 meta/classes/package.bbclass | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

index 2aec3e6..d7c970b 100644
@@ -568,7 +568,10 @@ python fixup_perms () {
             elif gid.isdigit():
                 return int(gid)
             else:
-                return grp.getgrnam(gid).gr_gid
+                try:
+                    return grp.getgrnam(gid).gr_gid
+                except Exception:
+                   return -1
 
         # Use for debugging the entries
         def __str__(self):

2.0.4