Another skiplist bug!

[Bron Gondwana]

Believe it or not, I'm actually not perfect, and there's a doozy
of a bug in skiplists that I'm probably entirely responsible for.

If your foreach callback function returns a non-zero result, then
it double-unlocks.  Wow, what a pain.

Attached patch fixes it.  I've run it on my testbed happily, and
will roll it out to production tomorrow.

Bron ( releases by embarassment - I think it's getting on time for
       a 2.3.15 if we can round up all our bugfixes )

diff --git a/lib/cyrusdb_skiplist.c b/lib/cyrusdb_skiplist.c
index 36110ca..5b481d4 100644
--- a/lib/cyrusdb_skiplist.c
+++ b/lib/cyrusdb_skiplist.c
@@ -1070,6 +1070,7 @@ int myforeach(struct db *db,
     size_t savebuflen = 0;
     size_t savebufsize;
     int r = 0, cb_r = 0;
+    int need_unlock = 0;
 
     assert(db != NULL);
     assert(prefixlen >= 0);
@@ -1093,6 +1094,7 @@ int myforeach(struct db *db,
        if ((r = read_lock(db)) < 0) {
            return r;
        }
+       need_unlock = 1;
     } 
 
     ptr = find_node(db, prefix, prefixlen, 0);
@@ -1112,6 +1114,7 @@ int myforeach(struct db *db,
                if ((r = unlock(db)) < 0) {
                    return r;
                }
+               need_unlock = 0;
            }
 
            /* save KEY, KEYLEN */
@@ -1131,6 +1134,7 @@ int myforeach(struct db *db,
                if ((r = read_lock(db)) < 0) {
                    return r;
                }
+               need_unlock = 1;
            } else {
                /* make sure we're up to date */
                update_lock(db, *tidptr);
@@ -1161,7 +1165,7 @@ int myforeach(struct db *db,
        }
     }
 
-    if (!tidptr) {
+    if (need_unlock) {
        /* release read lock */
        if ((r = unlock(db)) < 0) {
            return r;