Performance improvements to the (debug-use only) Mem.pScopyFrom logic, resulting

in about 8x faster performance under -DSQLITE_DEBUG for the query in from
[forum:/forumpost/0025389d0860af82|forum post 0025389d0860af82].  This change
only affects builds that use -DSQLITE_DEBUG.

FossilOrigin-Name: 7fb1ae25d1572dd7709a0f850c148a5f745a524f01ea231e29a1ebc37c173fb9

src/vdbeapi.c

@@ -1294,6 +1294,7 @@ static const Mem *columnNullValue(void){
 #ifdef SQLITE_DEBUG
         /* .pScopyFrom = */ (Mem*)0,
         /* .mScopyFlags= */ 0,
+        /* .bScopy     = */ 0,
 #endif
       };
   return &nullMem;