Make the sqlite3ExprCodeTarget() expression code generator routine robust
in the face of unknown opcodes - it simply generates a NULL value.

FossilOrigin-Name: f8e876c82a246ceed32b166f64e05dfe5ce4ab4c6820be60404109b43d36bb80
diff --git a/src/expr.c b/src/expr.c
index 3b2b513..c6920b3 100644
--- a/src/expr.c
+++ b/src/expr.c
@@ -3699,7 +3699,8 @@
       sqlite3VdbeLoadString(v, target, pExpr->u.zToken);
       return target;
     }
-    case TK_NULL: {
+    default: {
+      testcase( op!=TK_NULL );
       sqlite3VdbeAddOp2(v, OP_Null, 0, target);
       return target;
     }
@@ -4210,7 +4211,7 @@
     ** or if there is no matching Ei, the ELSE term Y, or if there is
     ** no ELSE term, NULL.
     */
-    default: assert( op==TK_CASE ); {
+    case TK_CASE: {
       int endLabel;                     /* GOTO label for end of CASE stmt */
       int nextCase;                     /* GOTO label for next WHEN clause */
       int nExpr;                        /* 2x number of WHEN terms */