[sql] Check for open db before releasing cache.

sql/connection.cc

Index: sql/connection.cc
diff --git a/sql/connection.cc b/sql/connection.cc
index 662127c2796d67112ef9ccfff4d42d6a188f6b03..20f0f075cbafb0dd6c09f4afe5f4a021f5522b36 100644
--- a/sql/connection.cc
+++ b/sql/connection.cc
@@ -512,6 +512,8 @@ void Connection::Preload() {
 // work.  There could be two prepared statements, one for cache_size=1 one for
 // cache_size=goal.
 void Connection::ReleaseCacheMemoryIfNeeded(bool implicit_change_performed) {
+  DCHECK(is_open());
+
   // If memory-mapping is not enabled, the page cache helps performance.
   if (!mmap_enabled_)
     return;
@@ -1364,8 +1366,10 @@ void Connection::DoRollback() {
   RecordUpdateTime(delta);
   RecordOneEvent(EVENT_ROLLBACK);
 
-  // The cache may have been accumulating dirty pages for commit.
-  ReleaseCacheMemoryIfNeeded(false);
+  // The cache may have been accumulating dirty pages for commit.  Note that in
+  // some cases sql::Transaction can fire rollback after a database is closed.
+  if (is_open())
+    ReleaseCacheMemoryIfNeeded(false);
 
   needs_rollback_ = false;
 }