btree/bt_seq.c: Fix two NULL pointer dereferences

This change fixes two NULL pointer dereferences caused by the
__bt_first function.

The first was caused by returning 0 (i.e., RET_SUCCESS) when a key
was not found, causing the caller to dereference an uninitalized
or NULL pointer. The second one was caused by an if statment clobbering
a local variable with a function call result that might be NULL.

Reported by:	clang-tidy
Sponsored by:	Klara, Inc.
Reviewed by:	markj
Obtained from:	https://github.com/apple-oss-distributions/libc (partially)
Differential Revision:	https://reviews.freebsd.org/D54905

lib/libc/db/btree/bt_seq.c

@@ -325,7 +325,7 @@ usecurrent:		F_CLR(c, CURS_AFTER | CURS_BEFORE);
 static int
 __bt_first(BTREE *t, const DBT *key, EPG *erval, int *exactp)
 {
-	PAGE *h;
+	PAGE *h, *hprev;
 	EPG *ep, save;
 	pgno_t pg;
 
@@ -338,7 +338,7 @@ __bt_first(BTREE *t, const DBT *key, EPG *erval, int *exactp)
 	 * page) and return it.
 	 */
 	if ((ep = __bt_search(t, key, exactp)) == NULL)
-		return (0);
+		return (RET_SPECIAL);
 	if (*exactp) {
 		if (F_ISSET(t, B_NODUPS)) {
 			*erval = *ep;
@@ -369,14 +369,14 @@ __bt_first(BTREE *t, const DBT *key, EPG *erval, int *exactp)
 					break;
 				if (h->pgno != save.page->pgno)
 					mpool_put(t->bt_mp, h, 0);
-				if ((h = mpool_get(t->bt_mp,
+				if ((hprev = mpool_get(t->bt_mp,
 				    h->prevpg, 0)) == NULL) {
 					if (h->pgno == save.page->pgno)
 						mpool_put(t->bt_mp,
 						    save.page, 0);
 					return (RET_ERROR);
 				}
-				ep->page = h;
+				ep->page = h = hprev;
 				ep->index = NEXTINDEX(h);
 			}
 			--ep->index;