Fixes a race condition in CreateNewBlock.

CreateNewBlock was reading pindexBest at the start before taking the lock
so it was possible to have the the block content not match the prevheader.

(Partial of faff50d129)

src/main.cpp

@@ -3032,7 +3032,7 @@ public:
 
 CBlock* CreateNewBlock(CReserveKey& reservekey)
 {
-    CBlockIndex* pindexPrev = pindexBest;
+    CBlockIndex* pindexPrev;
 
     // Create new block
     auto_ptr<CBlock> pblock(new CBlock());
@@ -3054,6 +3054,7 @@ CBlock* CreateNewBlock(CReserveKey& reservekey)
     CRITICAL_BLOCK(cs_main)
     CRITICAL_BLOCK(cs_mapTransactions)
     {
+        pindexPrev = pindexBest;
         CTxDB txdb("r");
 
         // Priority order to process transactions