Add an assert() that verifies that the statement pointer from

the sqlite3_prepare() family of functions is always NULL if the routines
return other than SQLITE_OK.

FossilOrigin-Name: 0bb1cfc63f982db7b29c8e6be6698a6dad100f70

src/prepare.c

@@ -706,6 +706,7 @@ static int sqlite3LockAndPrepare(
   }
   sqlite3BtreeLeaveAll(db);
   sqlite3_mutex_leave(db->mutex);
+  assert( rc==SQLITE_OK || *ppStmt==0 );
   return rc;
 }