Auto merge of #2440 - bitcartel:1360_reduce_info_leakage_vpub_new, r=str4d

Fix intermediate vpub_new leakage in multi joinsplit tx

Resolves information leakage discussed here https://github.com/zcash/zcash/issues/1360#issuecomment-284561331

Example output in debug.log showing how intermediate vpub_new is now 0
https://gist.github.com/bitcartel/86ca316b74f0b1ec6d8de3ac132006da

When reviewing, note that the indentation of a large chunk of code (scenario 3 part 2) changed, so use `git diff -w` (or similar) or append `?w=1` to the Github URL e.g. https://github.com/zcash/zcash/pull/2440/files?w=1