Initialize pindexNewTip at declaration time
Initialize pindexNewTip at declaration time

ths fix this warning:

main.cpp: In function 'bool ActivateBestChain(CValidationState&, const
CChainParams&, const CBlock*, bool)':
main.cpp:3504:59: warning: 'pindexNewTip' may be used uninitialized in
this function [-Wmaybe-uninitialized]
             uiInterface.NotifyBlockTip(true, pindexNewTip)

Such warning pops up only while using gcc 5.3.0, 4.8.x and 6.3.0
don't emit anything even if the warning is legit.

(thanks to awemany for the help in tracking down which gcc version
were affected)

Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,Justaphf/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,Justaphf/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited