Only release the connection if we didn't report it as failed.

src/java/com/samskivert/depot/PersistenceContext.java

@@ -570,9 +570,11 @@ public class PersistenceContext
                 }
 
             } finally {
+                if (conn != null) {
                     _conprov.releaseConnection(_ident, isReadOnly, conn);
                 }
             }
+        }
 
         // if we got here, we want to retry a transient failure
         return invoke(op, false);