Fix a segfault that could occur following an OOM condition.

FossilOrigin-Name: 9adf6e2469d18bc3bfc0c804cfcaa692e23ab6b3e13465dcfc51c4b111b05cb4

test/notnullfault.test

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+# 2021 February 15
+#
+# The author disclaims copyright to this source code.  In place of
+# a legal notice, here is a blessing:
+#
+#    May you do good and not evil.
+#    May you find forgiveness for yourself and forgive others.
+#    May you share freely, never taking more than you give.
+#
+#***********************************************************************
+# This file implements regression tests for SQLite library.  The
+# focus of this file is testing optimizations associated with "IS NULL"
+# and "IS NOT NULL" operators on columns with NOT NULL constraints.
+#
+
+set testdir [file dirname $argv0]
+source $testdir/tester.tcl
+set testprefix notnullfault
+
+do_execsql_test 1.0 {
+  CREATE TABLE t1(a, b);
+  CREATE TABLE t2(c, d NOT NULL);
+}
+faultsim_save_and_close
+
+do_faultsim_test 1 -prep {
+  faultsim_restore_and_reopen
+} -body {
+  execsql {
+    SELECT * FROM t2 WHERE d NOT NULL
+  }
+} -test {
+  faultsim_test_result {0 {}}
+}
+
+#-------------------------------------------------------------------------
+reset_db
+do_execsql_test 2.0 {
+  CREATE TABLE t1(a, b, c); 
+  CREATE TABLE t2(a, b, c, PRIMARY KEY(a, b, c)) WITHOUT ROWID;
+}
+faultsim_save_and_close
+
+do_faultsim_test 2.1 -faults oom-t* -prep {
+  faultsim_restore_and_reopen
+} -body {
+  execsql {
+    SELECT dense_rank() OVER win FROM t2
+    WINDOW win AS (ORDER BY c IS NULL)
+  }
+} -test {
+  faultsim_test_result {0 {}}
+}
+
+finish_test
+