Check block header before accepting it.

Previously, AcceptBlockHeader did not check the header (in particular
PoW).  This made the client accept invalid-PoW-headers from peers in
headers-first sync.

src/main.cpp

@@ -2280,6 +2280,8 @@ bool CheckBlock(const CBlock& block, CValidationState& state, bool fCheckPOW, bo
 {
     // These are checks that are independent of context.
 
+    // Check that the header is valid (particularly PoW).  This is mostly
+    // redundant with the call in AcceptBlockHeader.
     if (!CheckBlockHeader(block, state, fCheckPOW))
         return false;
 
@@ -2351,6 +2353,9 @@ bool AcceptBlockHeader(const CBlockHeader& block, CValidationState& state, CBloc
         return true;
     }
 
+    if (!CheckBlockHeader(block, state))
+        return false;
+
     // Get prev block index
     CBlockIndex* pindexPrev = NULL;
     int nHeight = 0;