libsemanage: Always set errno to 0 before calling getpwent()

The manpage explicitly states that:

  The  getpwent()  function  returns a pointer to a passwd structure, or
  NULL if there are no more entries or an error occurred.  If an error
  occurs, errno is set appropriately.  If one wants to check errno after
  the call, it should be set to zero before the call.

Without this, genhomedircon can wrongly return the following:
  libsemanage.get_home_dirs: Error while fetching users.  Returning list so far.

SELinuxProject#121

Signed-off-by: Laurent Bigonville <bigon@bigon.be>

libsemanage/src/genhomedircon.c

@@ -361,7 +361,11 @@ static semanage_list_t *get_home_dirs(genhomedircon_settings_t * s)
 
 	errno = 0;
 	setpwent();
-	while ((pwbuf = getpwent()) != NULL) {
+	while (1) {
+		errno = 0;
+		pwbuf = getpwent();
+		if (pwbuf == NULL)
+			break;
 		if (pwbuf->pw_uid < minuid || pwbuf->pw_uid > maxuid)
 			continue;
 		if (!semanage_list_find(shells, pwbuf->pw_shell))
@@ -403,7 +407,6 @@ static semanage_list_t *get_home_dirs(genhomedircon_settings_t * s)
 		}
 		free(path);
 		path = NULL;
-		errno = 0;
 	}
 
 	if (errno) {
@@ -1101,7 +1104,11 @@ static int get_group_users(genhomedircon_settings_t * s,
 	}
 
 	setpwent();
-	while ((pw = getpwent()) != NULL) {
+	while (1) {
+		errno = 0;
+		pw = getpwent();
+		if (pw == NULL)
+			break;
 		// skip users who also have this group as their
 		// primary group
 		if (lfind(pw->pw_name, group->gr_mem, &nmembers,