Make the sqlite3ExprCodeTarget() expression code generator routine robust

in the face of unknown opcodes - it simply generates a NULL value.

FossilOrigin-Name: f8e876c82a246ceed32b166f64e05dfe5ce4ab4c6820be60404109b43d36bb80

manifest

@@ -1,5 +1,5 @@
-C Fix\sproblems\sin\sfts3\swith\sprocessing\scorrupt\srecords\sand\sundefined\sinteger\soverflows.
-D 2019-12-14T14:15:21.800
+C Make\sthe\ssqlite3ExprCodeTarget()\sexpression\scode\sgenerator\sroutine\srobust\nin\sthe\sface\sof\sunknown\sopcodes\s-\sit\ssimply\sgenerates\sa\sNULL\svalue.
+D 2019-12-14T15:01:55.918
 F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
 F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
 F LICENSE.md df5091916dbb40e6e9686186587125e1b2ff51f022cc334e886c19a0e9982724
@@ -479,7 +479,7 @@ F src/date.c e1d8ac7102f3f283e63e13867acb0efa33861cf34f0faf4cdbaf9fa7a1eb7041
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 F src/delete.c a5c59b9c0251cf7682bc52af0d64f09b1aefc6781a63592c8f1136f7b73c66e4
-F src/expr.c 947911a1826673add2311e8650aeaf9c2809706dfaa45fc2445a5ff32f70693d
+F src/expr.c 530df5b8b684f94f0217115da30d292964385a83e87945bd9d7d792b08eb7d17
 F src/fault.c 460f3e55994363812d9d60844b2a6de88826e007
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 F src/func.c ed33e38cd642058182a31a3f518f2e34f4bbe53aa483335705c153c4d3e50b12
@@ -1852,7 +1852,7 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
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-U dan
-Z 2fcdc36e2d58b6d19eb97fe22b33d40d
+P 3b873029ef1903f76442445343af3786cd59ee66b0e5e0e6b76a0b6b1b5eb88a
+R 85c383a5f778e584f7e14589390faee1
+U drh
+Z d36aae0806eadd62df0ee61e4708a6ba

manifest.uuid

@@ -1 +1 @@
-3b873029ef1903f76442445343af3786cd59ee66b0e5e0e6b76a0b6b1b5eb88a
+f8e876c82a246ceed32b166f64e05dfe5ce4ab4c6820be60404109b43d36bb80

src/expr.c

@@ -3699,7 +3699,8 @@ expr_code_doover:
       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 @@ expr_code_doover:
     ** 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 */