modified blockMinter.java to check for null peers (thanks @kennycud)

[crowetic]

This change is to prevent NullPointerException and blockMinter thread crash.

The current theory is that nodes that have been running a long time have a higher likelihood of having a crashed blockMinter thread, thus preventing them from minting new blocks. Theory is that this is caused by nefarious nodes shutting their nodes down at the perfect time to allow a null peer to get into the peers list, and upon attempting to check for a higher weight block, a null pointer exception takes place, thus crashing the block minter.

When this happens, the block minter stays crashed, and doesn't fix itself.

We are looking into longer-term changes that will both more thoroughly handle the issue, and check the block minter for crashes and restart it, but for now this change should hopefully prevent a null peer from causing the null pointer, and thus prevent the block minter from crashing.

Thanks to @kennycud for the lesson and initially tracking down the issue!