Improved assert()s on the sqlite3ExprListDup() logic for TK_SELECT_COLUMN.

FossilOrigin-Name: 14da99d41f7968bf816203b4ae11c1f0d1ee0b5d
diff --git a/manifest b/manifest
index b47ade3..d80ba5d 100644
--- a/manifest
+++ b/manifest
@@ -1,5 +1,5 @@
-C Test\scases\sfor\sthe\srow-value\sUPDATE\sTRIGGER\sfix.
-D 2017-01-02T23:43:03.976
+C Improved\sassert()s\son\sthe\ssqlite3ExprListDup()\slogic\sfor\sTK_SELECT_COLUMN.
+D 2017-01-03T00:27:16.468
 F Makefile.in 41bd4cad981487345c4a84081074bcdb876e4b2e
 F Makefile.linux-gcc 7bc79876b875010e8c8f9502eb935ca92aa3c434
 F Makefile.msc b8ca53350ae545e3562403d5da2a69cec79308da
@@ -341,7 +341,7 @@
 F src/date.c dc3f1391d9297f8c748132813aaffcb117090d6e
 F src/dbstat.c 19ee7a4e89979d4df8e44cfac7a8f905ec89b77d
 F src/delete.c c8bc10d145c9666a34ae906250326fdaa8d58fa5
-F src/expr.c c2be1c1ae369dc14ba9e76831466b3a494986872
+F src/expr.c e115097aa866a462051ac8430757638dc1d73f1c
 F src/fault.c 460f3e55994363812d9d60844b2a6de88826e007
 F src/fkey.c 2e9aabe1aee76273aff8a84ee92c464e095400ae
 F src/func.c d8582ee91975975645f206db332c38f534b783ad
@@ -1541,7 +1541,7 @@
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P 2caaaab5e07f6b76d66fbe4e93a4b956937427d8
-R 1083f74702d2e1110eb1d37d8553d935
+P b9b964373c89a491649f366ad7d70ae4d69b891d
+R b890811ca8348b8ceda7efec0ae53743
 U drh
-Z d43bfd5db806552d45260c4ffdcc3bd0
+Z f487608154c66bed79f05de2b11f93c4
diff --git a/manifest.uuid b/manifest.uuid
index 1c1f56c..313a7e2 100644
--- a/manifest.uuid
+++ b/manifest.uuid
@@ -1 +1 @@
-b9b964373c89a491649f366ad7d70ae4d69b891d
\ No newline at end of file
+14da99d41f7968bf816203b4ae11c1f0d1ee0b5d
\ No newline at end of file
diff --git a/src/expr.c b/src/expr.c
index 072c420..6d4b48f 100644
--- a/src/expr.c
+++ b/src/expr.c
@@ -1295,6 +1295,7 @@
   ExprList *pNew;
   struct ExprList_item *pItem, *pOldItem;
   int i;
+  Expr *pPriorSelectCol = 0;
   assert( db!=0 );
   if( p==0 ) return 0;
   pNew = sqlite3DbMallocRawNN(db, sizeof(*pNew) );
@@ -1311,7 +1312,6 @@
     Expr *pOldExpr = pOldItem->pExpr;
     Expr *pNewExpr;
     pItem->pExpr = sqlite3ExprDup(db, pOldExpr, flags);
-    assert( pItem->pExpr==0 || db->mallocFailed==0 );
     if( pOldExpr 
      && pOldExpr->op==TK_SELECT_COLUMN
      && (pNewExpr = pItem->pExpr)!=0 
@@ -1319,9 +1319,13 @@
       assert( pNewExpr->iColumn==0 || i>0 );
       if( pNewExpr->iColumn==0 ){
         assert( pOldExpr->pLeft==pOldExpr->pRight );
-        pNewExpr->pLeft = pNewExpr->pRight;
-      }else if( ALWAYS(pItem[-1].pExpr!=0) ){
-        pNewExpr->pLeft = pItem[-1].pExpr->pLeft;
+        pPriorSelectCol = pNewExpr->pLeft = pNewExpr->pRight;
+      }else{
+        assert( i>0 );
+        assert( pItem[-1].pExpr!=0 );
+        assert( pNewExpr->iColumn==pItem[-1].pExpr->iColumn+1 );
+        assert( pPriorSelectCol==pItem[-1].pExpr->pLeft );
+        pNewExpr->pLeft = pPriorSelectCol;
       }
     }
     pItem->zName = sqlite3DbStrDup(db, pOldItem->zName);