[2.23 COMMITTED] nss_db: Fix initialization of iteration position [BZ #20237]

[fweimer@redhat.com (Florian Weimer)]

When get*ent is called without a preceding set*ent, we need
to set the initial iteration position in get*ent.

Reproducer: Add “services: db files” to /etc/nsswitch.conf, then run
“perl -e getservent”.  It will segfault before this change, and exit
silently after it.

(cherry picked from commit 31d0a4fa646db8b8c97ce24e0ec0a7b73de4fca1)

2016-06-11  Florian Weimer  <fweimer@redhat.com>

	[BZ #20237]
	* nss/nss_db/db-XXX.c (set*ent): Reset entidx to NULL.
	(get*ent): Set entidx to NULL during initialization.  If entidx is
	NULL, start iteration from the beginning.

diff --git a/nss/nss_db/db-XXX.c b/nss/nss_db/db-XXX.c
index 125a5e9..2d13edd 100644
--- a/nss/nss_db/db-XXX.c
+++ b/nss/nss_db/db-XXX.c
@@ -77,7 +77,7 @@ CONCAT(_nss_db_set,ENTNAME) (int stayopen)
       keep_db |= stayopen;
 
       /* Reset the sequential index.  */
-      entidx  = (const char *) state.header + state.header->valstroffset;
+      entidx  = NULL;
     }
 
   __libc_lock_unlock (lock);
@@ -253,8 +253,14 @@ CONCAT(_nss_db_get,ENTNAME_r) (struct STRUCTURE *result, char *buffer,
 	  H_ERRNO_SET (NETDB_INTERNAL);
 	  goto out;
 	}
+      entidx = NULL;
     }
 
+  /* Start from the beginning if freshly initialized or reset
+     requested by set*ent.  */
+  if (entidx == NULL)
+    entidx = (const char *) state.header + state.header->valstroffset;
+
   status = NSS_STATUS_UNAVAIL;
   if (state.header != MAP_FAILED)
     {