db: fixed space leak in statement finalizer thread

This "fixes" the segfault in the test program, but it seems
like there must be another underlying problem.

collects/db/private/generic/prepared.rkt

@@ -109,5 +109,5 @@
                       [else
                        "prepared statement finalizer thread handled non-exception"])
                 e))])
-         (will-execute will-executor)
-         (loop))))))
+         (will-execute will-executor))
+       (loop)))))

collects/tests/db/programs/sl-inserts.rkt

@@ -0,0 +1,20 @@
+#lang racket
+(require db)
+
+(define db (sqlite3-connect #:database 'memory))
+
+(query-exec
+ db
+ "CREATE TABLE IF NOT EXISTS
+        log_word_map(word TEXT, log_id INTEGER,
+            PRIMARY KEY (word, log_id)
+            ON CONFLICT FAIL)")
+
+(query-exec db "BEGIN TRANSACTION")
+
+(for ([x (in-range 20000)])
+  (query-exec db
+   "insert into log_word_map values (?, ?)"
+   (number->string x) 99))
+
+(query-exec db "COMMIT")