nss_db: reset mapping after last get*ent

[DJ Delorie]

nss_db allows for getpwent et al to be called without a set*ent,
but it only works once.  After the last get*ent a set*ent is
required to restart, because the end*ent did not properly reset
the module.  Resetting it to NULL allows for a proper restart.

	* nss/nss_db/db-open.c (internal_endent): Reset mapping to NULL.

diff --git a/nss/nss_db/db-open.c b/nss/nss_db/db-open.c
index 8a83d6b930..f7c53b4486 100644
--- a/nss/nss_db/db-open.c
+++ b/nss/nss_db/db-open.c
@@ -64,4 +64,5 @@ void
 internal_endent (struct nss_db_map *mapping)
 {
   munmap (mapping->header, mapping->len);
+  mapping->header = NULL;
 }